Won't be oil pressure as this is part of the car loom at least it is on other Sierras can't see they changed it for 4x4

I would think that is something unique to 4x4 loom like A/C perhaps or something as I have never seen it on a 2WD loom.

The pipe you are likely seeing is a take off for a dump valve etc. You should have another one the same which the fuel pressure regulator connects to, unless you mean underneath the plenum where there is an outlet for a breather.

The square type on the right are for the CTS, Crankshaft Position sensor (TDC) and ACT, third is most likely knock sensor.

The TPS and MAP sensor are the ones in the middle by the looks of it, the TPS being the red one and the black one above for the MAP sensor (These have three wires so are impossible to confuse with the aforementioned sensors).

Martin