The way I reconcile C3-PO in my head is that Anakin assembled 3po more than he built him , I like the old lore that he was built on Affa or some such planet 125 years before the original Star Wars . I imagine he got in some kind of trouble and was disassembled and left in a junk heap on Tatooine and Anakin just found the parts and put him back together.
Well, that is literally what “happened” in Legends canon. I can’t remember the details, but there was a comic where Vader finds C-3PO’s head on Cloud City and then has a flashback to finding his disassembled parts (I think someone used him as target practice) in Mos Espa and rebuilding him.