What happened to the technical drawings of old and obsolete vehicles, machines?

This is often discussed with "why can't we make Saturn V again" flavour. People dispute how much of the inputs are still available, other people decry even trying because some things (the F1) were essentially iterations in the field and may not have made it back into the tech drawings beyond "modify the bend radius until engine stops vibrating" annotations.

Around WW2 American aircraft which ditched in Soviet controlled territory were taken apart. There is a story in Richard Rhodes' books on the A bomb and H bomb that they re-created mistaken holes, welded repair plates into brand new aircraft in a chinese-copy mentality where they lacked time to make decisions what was needed and what was not. This allowed them to field long range bombers.

Some things only require physics to be proved. Soviet spying informed how to make Atom Bombs, even when all they had was measured constraints rather than drawings.

Again from Rhodes, they worked blind, the "proof" was when their science agreed with what they were told the Manhattan project had found. If they diverged and couldn't justify it, they had to re-do their working until they aligned.

Conduct a patent search[1], find out who owns the rights / has used the patent rights and/or other information for tracking down technical specifications. aka Where to request technical drawings.

If initially was a research project, might be a thesis paper(s) / research documentation on what looking for.

[1] : https://www.uspto.gov/patents/search