Differences between upscaling methods. What is DLSS?

I am not sure I understand what DLSS is. It feels like everyone on yt is praising it, but don’t really explain it.

So far I know that it is a third party upscaling method from Nvidia, and it is running the game in a lower resolution and then using AI to scale it larger but making the resolution look as good, which keeps the fps / makes better fps (correct me if I’m wrong).

But what’s the difference between using UE’s TSR to Nvidia’s DLSS, or even AMD’s FSR or Intel’s XeSS?