Linux

this distribution allows you to have multiple versions of the same library to work with different programs at the same time, correct?

That is correct. You can even have programs from entirely different releases running on the same system without conflicting with another; one with i.e. glibc 3.34 and one with 3.37 for example. Or even wilder setups with some packages using musl, other glibc and others yet being built statically.

Does this mean that each program downloads all its dependencies independently?

Each program references all of its (exact) dependencies. Nix then looks at the program and its references and builds a tree of dependencies.
Then it tries to "realise" these dependencies (make them, well, real), possibly by substituting (downloading) them from a binary cache or automatically building them on your machine if they're not available in any configured cache.

However, if some package with the same exact version already exists in the Nix store, no action will be taken. Why should it, it's already there.
For example, if you were in an empty world and built an environment with hello and coreutils in it, they'd both depend on glibc. If both came from the same revision of Nixpkgs, chances are that they depend on the exact same version of glibc.
What Nix would do here is fetch 1x glibc, 1x hello and 1x coreutils.

Note however that you don't need to manage any of this. You just say "I want hello and coreutils". Nix takes care of getting the correct dependencies in place but they won't be in your immediate environment. When you then say that your environment should no longer contain hello, it does that. At this point hello will still exist in the Nix store but it won't be in your PATH any longer, so it's not polluting any shared state; it just sits there on disk and the worst it could do is waste disk space. It's not "installed" in the same sense as what it'd mean to have an unused dependencies installed on an FHS system.
In order to reclaim disk space from unused Nix store paths, you can simply run a garbage collection. You don't need to care about that one specific hello store path here though, you just say "remove all unused store paths for me, thanks" and Nix removes it along with all other unnecessary paths. In NixOS, you can even have that ran periodically for you.
(Note that this is distinct from autoremove and the like; those "clean up" the shared state and free up disk space. In NixOS, these are separate processes and dependencies which you don't explicitly declare are never in the shared state to begin with.)

You see, while this could be seen as "bloat", it has none of the negative consequences bloat has on other systems such as more packages for you to manually manage, more binaries in your PATH or weird interactions of other programs. It's just easily managed disk space and disk space is honestly quite cheap.

Not exactly. If one package needs foo as a dependency, and another package also needs foo, it won't download a second copy.

But if another package needs, say, an older version of foo, it can download the older version of foo and you'll have two foo packages.

This is possible because Nix packages have hashes to differentiate between them, so any package can ask for the exact version of foo it needs instead of asking for foo in general and hoping the version it gets isn't incompatible.

Pretty sure this is also different from other containerized package managers, like Flatpaks or Snaps, which I believe throw all the dependencies in with their packages so each package has exactly what it needs in its container, and which is obviously going to be much more bloated than having shared dependencies. As far as I know Nix on the other hand doesn't get any more bloated than other distros (keep in mind that regular distros like Arch will still sometimes have multiple versions of a package, for example Python).

Yeah. I feel the same way. I remember NPM (nodejs) had this exact issue and you would end up having so many duplicate packages.

I believe shared dependencies are not duplicated.

Each version of a library (or any package) will only exist once, and things are garbage collected when not referenced.

Tbh i would not mind the extra bloat if it meant i get stability.