There does not seem to be a definitive answer to how proteins destined to be intrinsic plasma membrane proteins are directed there. Presumably, assuming starting at a cytosolic ribosome, the pathway is via the SRP/Sec61 translocon system into the ER co-translationally, followed by COPII vesicular transport to the Golgi for modifications.
From here, is the system that of COPI transport back to the ER? I think I read that if sitting in the ER membrane, proteins can 'slide' into the PM via some vague endomembrane system juggling. Or, packaging into vesicles which can then fuse with the PM?