How is upscale FHD to 4K different from normal FHD output to a 4K display?

Question

Video

I know there are different upscaling algorithms, but I'm more interested in why they should be better than just outputting FHD to a 4K panel. Moreover, those that manage to work realtime simply smooth the image, and do not finish the missing one.
If anything, I did not consider live 4K screens.

Answer 1

"simple FHD output at 4k" is an FHD picture in the center and black borders around the edges.
everything else is upscaling, that is, stretching by various algorithms to a higher resolution.
and already in the algorithms and the whole point. from the banal "nearest neighbor" to complex super resolution.