V’Ger wasn’t built by humanity it was built around a fictional Voyager spacecraft, who built it was never disclosed by the common fandom theory is that this might have been the progenitor idea to what eventually became the Borg in TNG.

Hmm, Memory Alpha does confirm your facts. I never realized that the remainder of V'Ger had been "constructed" by someone else. I had just assumed that it had ... spontaneously grown and developed out of the Voyager VI core built by humans.

Where did this backstory come from? Certainly it was not treated in the original theatrical cut, where Kirk only finds its true nature toward the very end. I certainly don't remember anyone being able to trace the history of V'Ger with any certainty beyond finding its original inscriptions.

No idea in the movie the original voyager is there almost as an altar I don’t even recall if it was operational or not.

As to whom built it im guessing the theories came form fans and the writing crew Star Trek has had a lot of notes being collected over the years it’s not uncommon for future shows to pick up on ideas that were thrown around the writing rooms from GR’s own notes and short stories and I’m guessing trough the years there been enough interviews for this to leak.

I’m not a big Trekkie so I don’t recall if anything other than that was in the movie I just remember it for all the butt jokes around the space anus and that voyager was at the heart of w/e vger was.