Years ago I tried Ubuntu which used GNOME and assumed that its desktop layout was “the default” GNOME.

Ubuntu never provided a "default" GNOME experience, or at least not since the early 2000s. At one point Ubuntu had it's own desktop environment, unity, and when they abandoned the project and switched to gnome shell, they modified it to look similar to that. So we went from this:

to this:

I later tried PopOS which also uses it and it was the same

I doubt that. Pop_OS was never the same as Ubuntu. In the beginning it provided an experience that was arguably much closer to vanilla gnome than Ubuntu:

later they started to add their own flare customising the desktop a lot and rebranding all of this as Cosmic Shell:

I installed Mint I saw that it’s still fundamentally the same

Mint never used Gnome. They have their own desktop environment called Cinnamon, which uses some of the same underlying technology, manly the GTK toolkit but is it's own independent project.

Well, few days ago I installed Bazzite (Fedora) which is also GNOME. It doesn’t look anything like anything I’ve seen before

It still should be familiar in some aspects. The grid-view of all apps for example should be something that you know from Ubuntu (Pop replaces that part in more modern iterations - kinda). Bazzite still does modifications, they're just a lot more subtle than the ones from Pop and Ubuntu. But I agree with you: Gnomes workflow doesn't agree with many users. There are those who like it and they tend to really like it. Gnome wants you to heavily use keyboard shortcuts and virtual desktops and I found that casual users aren't really a friend of either.

So what is the default Gnome experience? try it out. There is an extension manger installed that ships with bazzite (and if not use the software store to install it). In it you can disable all the modifications. You can also install extensions to fix some of the usability problems. "Dash to Dock" tends to solve like 90% of them (It's also what Ubuntu uses for their modifications)