cpu and gpu are welded on the motherboard?

"The GPU is soldered to the logic board this system uses DDR5 VRAM not HBM2 VRAM.

As for the CPU its socketed but you are limited to what CPU’s this series offered EveryMac - iMac19,1 systems

Sounds like you want to do some heavy graphics here. I would recommend you wait until after WWDC to see what Apple discloses."