[dasyatidprime]

Last I checked (which was quite a while ago), Android with multi-user support did in fact assign one Linux UID to each (user, app) tuple! But I don't recall there being a particularly rich privilege model available in practice for the multi-user sharing case, only for isolation. Inter-app intents were handled using Binder IPC underneath; I don't know what use that made of the Linux credentials.

Many server applications handle user separation internally, without reference to the underlying OS, while application separation is much stronger (separate VMs, SELinux, etc.), and desktop platforms have user separation but often-unsandboxed apps, so those are in some ways duals of each other…

I'm not sure what Windows does with UWP and sandboxed apps from the Microsoft Store, but that would be a good place to look.