I completely understand what you're saying about the misinformation. And I should say thanks for being that way. I'm glad you want to actually know how the whole thing works. I won't go through the whole thing in major detail because that could take forever, but I'll run though the basic idea and if you want to know more about any part of it, just say so
Ok, so I'll start with what happens when air enters the intake... Air goes into the intake and travels past two sensors. They are commonly referred to as the MAF sensor, but on our cars, they're actually two sensors in one housing. The MAF sensor is two little resistor filaments (tiny wires) that the ecu heats up to a certain temperature. As air travels through the intake, part of that air gets sent up through the sensor housing and hits those wires. That air cools the wires. The ecu tries to keep the wires at a constant temperature, so more voltage is sent through the wires. As it turns out, the mass of air coming in contact with the wires is directly related to the cooling effect that air has on the wires, and thusly the amount of power the ecu must send to the wires to keep them hot. That's the principal the MAF sensor uses to determine how much air is there. The ecu then takes this voltage reading and compares it to a pre-established MAF scalar value that's part of the ECU programming. It looks like a spreadsheet with two rows and a bunch of columns. The first row is the voltage the ECU needed to send to the sensor to keep the wires hot, and the second row is the corresponding airflow that's been programmed for that ECU map.
Also as part of that dual sensor setup is the Intake Air Temperature sensor. This is actually commonly mistaken as the MAF sensor itself. If you look at the sensor housing, the MAF sensor part is the two wires hidden up inside the vertical tube that most people don't even see. The thing they see is an orange bulb type thing that's right on the outside of the housing. That's actually the IAT sensor part. If you haven't guessed, that thing reads the temperature of the incoming air and allows the ECU to compensate for changes in temperature (which correspond to changes in oxygen density and is good to know to target fueling properly).
Next down the line is the inlet tube. The inlet tube has a number of connections that are related to the EVAP system (emissions related stuff). These are accessory and not absolutely necessary, so I'll skip them. There's also a PCV hose connection, but its not related to the inlet system directly, so I'll skip that too. Ok, next is the BPV return hose. This is where the air that's released by the BPV gets dumped back into the intake system. You should note that the position chosen to return the air is after the MAF sensor. This is because the air that is released by the BPV has already passed the MAF sensor, and has already been accounted for by the ECU. If, for example, the air is lost to the atmosphere (like when using an atmospheric BOV), the ECU will be expecting a mass of oxygen to be in the engine, but part of that never shows up. The ECU will fuel as if the oxygen was there, because the MAF sensor told the ECU that it would be there, but it got dumped to atmosphere, so it's never going to show up. That's what causes the rich condition related to the use of atmopheric BOVs... and why we say they are not generally a good idea. By dumping any excess air back into the system after the MAF sensor, the ECU never sees more or less air than the MAF sensor told it there would be. The system is a closed loop system, so the contents remain consistent and everything works the way it should.
Next on down the line is the turbo... or rather, the compressor side of the turbo. I'm going to assume everyone knows how a turbo works. There's two independent chambers connected by a rod with fan blades on both sides. When exhaust passes through the exhaust side, the blades on that side spin and because they're connected through the middle with a rod, the compressor blades also spin. This compresses the air received through the intake system and pushes that compressed air into the intercooler. The turbo is controlled by a system of vacuum hoses and a solenoid valve. It's helpful to note that prior to the turbo, the inlet system is always under vacuum. The turbo and engine are always sucking in air, so everything before the turbo is being sucked in. It's not until after the compressor wheel blades that the inlet air can be called "boost." Positive pressure is achieved when the compressor is pushing air into the intercooler faster than the engine can consume it. The ECU has a target for boost that's dependent on load and RPM. If you don't know what load is, just google it. It's an important concept when thinking about how an engine operates, but would take me a while to explain. Anyway, that target value is another thing that's changeable, but determined by the ECU map. When the ECU senses that target boost has been achieved using the MAP (manifold absolute pressure) sensor, it needs a way to tell the compressor to slow down and produce less boost. The way this is done is by allowing a solenoid valve to open which in turn allows the wastegate to open. Now, the wastegate is basically a door inside the turbo that allows exhaust gasses to go around the turbine wheel instead of past it. If the amount of exhaust going past the turbine wheel decreases, the pressure spinning that wheel decreases, and it slows down. That's true for the compressor side too, considering they are connected by that rotating shaft. So, in order to get the compressor side to stop producing more boost, the turbo just vents some of the exhaust gas around the turbine wheel. Simple enough, right? There's actually a lot more to that concept, but if you want to know more you can find an article published by Cobb Tuning (available on the forums and their website) that explains how the Subaru boost control strategy works.
Just as a side note, the ECU actually pulses the wastegate solenoid open and closed really really fast. This modulates the amount of air allowed to pass through the wastegate and around the turbine wheel. That's why you might see wastegate function referenced in WGDC (wastegate duty cycle). It's referenced as a 0-100% duty cycle of the wastegate solenoid, and thusly is a measure of how often the wastegate is open versus closed. Understanding that will help you understand what certain logs are saying about how the turbo is functioning as well as how to tune so more or less air passes the turbine (adjusting for over or under boost conditions).
Ok, so now you have pressurized air in the intercooler. Congrats
The purpose of the intercooler is to lower the temperature of that air. This is important because air pressure doesn't make power. Just having some high boost pressure doesn't mean that you will make good power. What matters is now much actual oxygen gets pushed into the cylinders. That oxygen can be thought of in terms of the number of oxygen molecules present and allowed to participate in combustion. Since hot air is less dense than cold air, and the volume of your intercooler and cylinders is a constant, the colder the air that's allowed into your engine, the more oxygen molecules there will be in the cylinders when it comes time for combustion. Mixed with the right amount of fuel, this will produce more power.
The reason an intercooler is useful is because the intake air that passed through the compressor side of the turbo has been heated up from the measurement that the IAT saw for incoming air. Two factors contribute to that extra heat... First, the turbo itself is hot. There's really no way around that because exhaust gasses are hot and the turbo needs them to spin. That thermal energy gets transferred to the intake air as well. The second source of thermal energy is the compression process itself. As you squeeze molecules closer and closer together, they vibrate more and bounce off eachother creating heat. If you want to know more about that, pick up any highschool chemistry book.
Now that there's pressurized air in the intercooler, everything will continue to go smoothly as air moves through the intercooler and past the throttle body into the intake manifold. This is true as long as the throttle body is open. What happens if the throttle body closes? We do this all the time when we lift our foot off the accelerator to make a shift. When that throttle plate snaps closed, the turbo compressor is still spinning and creating pressure, but the pressure has no place to go. That's why the BPV is there. The BPV is a valve that is set to open when the air pressure inside the intercooler exceeds the spring pressure of the BPVs spring/diaphragm assembly. This is all assisted much the same way as the wastegate is; by using vacuum pressure to offset the force needed to open the valve. When the throttle snaps closed suddenly, the pressure in the intercooler goes up past the point when the BPV can stay closed (past the spring pressure holding the BOVs diaphragm closed), and air is allowed to vent through the BPV return hose to the inlet tube after the MAF but before the turbo. That air is then allowed to go through the turbo again, get compressed again, and maybe make it into the cylinders for combustion.
The fun thing about cars is that the most simple concepts can be controlled with increasing levels of complexity. I know this might look like a lot of details about how air gets into your engine, but trust that there is a lot I have left out and a lot more to understand. On the surface, all a car needs is air, fuel, and spark. The cool part is measuring, metering, controlling those three basic ingredients. Let me know if I missed anything or if something needs to be clarified. It was good to hear someone wanted to understand and not perpetuate the misinformation plaguing the interwebz
