What resolution should a hand-drawn picture have to be used as the background of an iPad and iMAC game?

Question

Screen resolution

The iMac has the largest resolution now, it's 27 Retina - 5120x2880, so you need to draw a picture of the same resolution, and for the iPad you can simply reduce it to the desired resolution in the future - do I understand correctly?
In general, in what resolution should an artist draw in a raster?