I don't understand your question. The optical parts are in the lens. The lens, at least in most mirror cameras, whether using single or double-lens, small-, medium-, or large-format, are detachable. Most popular legacy mounts such as M42 can be used on modern DSLR and mirrorless systems with an adapter. Some, such as Nikon F-mount, don't even need an adapter. There are millions of images floating around taken this way, with varying technical/artistic quality.

Absolete ?

Million of people still use them both for personal and commercial use, especially medium/large format cameras. If anything they're the least likely cameras to get obsolete and the ones with the best proven long term usability. A 15 years old digital camera isn't really enjoyable anymore while a 40 years leica M or nikon FM are still mechanical beauties

There isn't much to repurpose, a film camera is basically a dark box with an advance lever and some form of shutter, can't really get much simpler than that. Most of them have interchangeable lenses which can be used on modern cameras very easily.

I'm still using my dad's 1978 Nikon FE every few days, the same that took my own childhood pictures. I'm actually selling my 2k$+ digital camera to go back to film only

Some links you might find interesting:

https://www.reddit.com/r/analog/

https://www.instagram.com/explore/tags/analog/

https://www.youtube.com/watch?v=Uizuh5S3bVE

Also, a mandatory note, that these are not obsolete, and will not be for a long time. Film is available, cameras are available and cheap, there still are services that develop those films for you and make printouts.

Some friends still use them with success, both Zenit units and more modern SLRs. Market is small but present.

you can and you should. I have a Zenitar 50mm f2, which was the standart glass fitted to the Zenit camera discussed in the article on a M42 adapter. It makes beautiful pictures especially considering ypu can pick it up for ~10€ or sometimes free.