Modern people overestimate how long we have been able to engineer things precisely.

I used to use a lot of vintage German cameras. Really right up through the 1950s the tolerances were wide enough that no two cameras were exactly alike. The parts were all replaceable, and there were often adjustments & shims inside to keep things in spec. For example, one camera might have part A that's a little long so it's matching part B would be a little short to compensate. They'd sort it out at the factory where they have the parts bins and could keep trying different part Bs to match the part A installed until they had a close enough match, then shim it.

This was even more so in the 1920s-1940s cameras.

The cameras actually had stickers in the film compartment warning you to send it in for professional service for any repairs. Note the implication that the camera needed frequent enough service that they'd actually bother putting a sticker in it!

So for some we look back on the era as things being user-serviceable and parts replaceable, but it was that way for reasons of manufacturing ability.