They apparently did a lot of the cgi in real time with an curved led wall and some version of unreal engine. They track the camera position so they can move the image on the screen to match properly and They only render high resolution what’s in the camera viewport and the other stuff just provides some nice realistic ambient light.
ILM video:
https://youtu.be/gUnxzVOs3rk
https://arstechnica.com/gaming/2020/02/the-mandalorian-was-s...