With BTTF 1, we see the START of the loop. Ever since then, Marty always went back to 1955, helped his parents get together, and then returned. That 2nd Marty at Lone
Pine Mall we see at the end of BTTF1 taking off with the Delorean, that's part of the never ending loop. That Marty had successful parents who met because of some mysterious guy called Marty helping them in 1955. So that 2nd Marty is trapped in a predestined loop.
But we, as the audience, always follow the very first Marty who started the predestination "paradox". He got the Delorean from a Doc Brown who never ever met Marty in 1955.