I'm wiring a subpanel in free standing garage.
70A circuit, on 4/3 conductor with a ground.

There is something that I don't understand:
The main panel has a bus that combinds the neutral and ground.
As I read about sub-panel wiring, the neutral and ground should be
seperate.

The seperate neutral and ground at the subpanel run to the combind bus
at the main panel, so electrically speaking, there is continuity
there..

Why wire them seperate at the subpanel?