They had a different look, but usage and UI conventions including keyboard navigation worked basically the same. Since the mixing of mobile and desktop with web technology, it’s the Wild West.

There was a short period of time around Windows 95-XP where application developers really wanted to try to use the "latest and greatest" because if they didn't they really stood out as being "last year's tech".

Then XP kind of broke that as so many started not bothering and then it all went sideways.

But they were not too far removed from how the standard Windows buttons looked. Office had its own look in a lot of ways back then, e.g. with the extra handles on the toolbars.

Now, if I take a couple of current applications, the differences are much bigger: from the top of my head, Explorer, Settings, Edge, Office, Steam, Spotify all have much more unique UIs.