Camera Tech Notes that explain some of what is discussed above (yes, you might be able to get a Minolta lens to at least sort of work with a mirrorless Sony...)
1. The distance from the back or base of the lens (the surface that presses up against the camera body) to the film plane is called the registration distance. If a lens is designed for say 30mm registration distance, and you somehow graft it onto a camera with a 32mm registration distance, that lens won't focus at infinity (*). But if you have a lens made to fit a camera with say a 29mm registration distance, and there's enough space, you could adapt it to a camera with a 27mm registration distance, and it will work fine.
(*) Very long focal length lenses (think 400mm +) sometimes have a lot of "overtravel" to account for thermal issues, and so some of them can overcome too-deep registration adapters, at least some of the time.
This is actually a bigger deal than the mechanical vs electronic coupling in going from old Canon mount to Canon EOS.
2. Throat diameter is just the size of the hole in the front of the camera that the lens projects light through - adapting a narrow throat lens to a wide throat camera is fairly easy - vice versa not so much.
3. Auto stop-down, auto focus, meter coupling. A fully manual camera (say a Leica M6 (film) or M10 (digital)) doesn't stop the lens down (doesn't have to), and meters "at working aperture", and has no autofocus - and so it has none of these. Due to a little bit of planning Leica screw mount lenses from before 1954 fit the film bayonet cameras and indeed the digital M series cameras up to right now.
Modern mirrorless digital cameras - Fuji X-mount, Canon R-mount, Nikon Z-mount, and Sony <whatever name> share this - so they don't actually need the meter-coupling. But they're all autofocus - but see below.
SLRs are more complicated. As correctly noted above the original Canon mount (I forget the name) depended on mechanical coupling cannot couple to the all electric EOS lens mount. But there's a weird out [1] below.
OK, so you can adapt from long-registration to short-registration, but you are likely housed on auto-stopdown/auto-focus coupling = BUT people have been using old lenses in fully manual for years (as in 5+ decades) - for things like macro lenses that was the norm anyway.
[1] OLD IS NEW:
If you are going to a *mirrorless* camera (ILC) - the Sony's, Fuji X-mount, Z-nikons and R-Canons. Or to a Leica M-10 for a different reason.
Why? Because all of those cameras are showing you a "TV" picture of what the lens sees, rather than having you peer through it optically - so there's no need to auto-stopdown the lens at all - just turn up the gain on the TV. (For the M-10 you end up using the preview screen on the back rather than the optical viewfinder.)
You end up with Manual Focus (the horror!) - but it can work.
Sometimes you have to change funky settings to persuade the camera to cooperate. Adapting anything to an EOS and I presume R mount is a notorious pain.
**BUT**
Why bother? If you are going to shell out the bucks for a Fuji-X, Canon R, Sony, Nikon Z, etc. - you probably want more modern lenses with autofocus. If you decide you want a Leica M10 (lovely cameras, but I sold mine) then you will be digging into lenses. And I have Stemar I'll sell you. (And yes it clears the M10 shutter...)
Happy Day