The 'I' in PID is the desired setpoint correct?
No.
The 'P' is the amount of overshoot and undershoot it takes to find the setpoint
No
'D' is what is responsible for learning patterns to reduce amount of over and undershoot to find the setpoint during fluctuating operation over time?
No
I have no idea where you are getting any of that from, since I can't find a single description of PIDs that says anything like any of the three.
Does anyone have a suuuuuuper laymen's description of this stuff?
I have no idea how to answer that. If you don't know what an integral or a derivative is, there is no way to answer that easily.
The wikipedia entry on PIDs is as good as it is going to get for you.
Is PID only responsible for part throttle boost conditions?
No, it is always active. Certainly you can see it doing something WOT, no?
Is the PID only put to use by the n75 or is the throttle used to meet PID output also?
Genearally n75, but the throttle will close (see also throttle cut, discussed in detail on the s4 wiki) if the PID detects a gross error (extended overshoot).
I did a WOT 3rd gear log the other day after restricting LDRXN to 7 PSI so It's less likely that I blow up my turbo. Fuel is still off when I thought I had it dialed in. It goes off course from desired AFR at high rpms and kind of sways around a bit before that. Not sure if BTS or AWKR is responsible as I am not fueling through LAMFA anymore.
You can log both to find out but tabgm_w should pretty much answer that question for you.