Linux

Where is the closed source user space of Intel and AMD drivers?

They're not in user space, they're in the firmware of the GPUs. It's embedded in some chip somewhere on the card or in the motherboard. The open source components communicate with that closed part.

Nvidia previously implemented nearly everything in their nonfree kernel module driver. Today, they've pushed enough of the parts they're protective of into the firmware, so that they can release the kernel module as open source/GPL.

they use Mesa for the best possible compatibility.

Mesa is just the userspace implementation of higher level graphics APIs like OpenGL or Vulkan, which communicate with the underlying drivers. I actually think its a good thing the Nvidia has their own implementation of this as it creates competition, and they're positioned to improve consistency across windows/Linux since they likely reuse a lot of code on both platforms.

I've read comments by people bashing the recent Baldur’s Gate 3 Linux release and being full of graphics glitches. Then they list their hardware as proof how great it is and they all have NVidia GPUs.

That's Larian's fault for releasing a buggy port. They probably only tested on AMD because they only care about the Steam Deck on Linux. GPU drivers are always buggy, even on windows. The only way to ensure compatibility is to spend the time and effort to test on all of them.