[nine_k]

It can't be anything else. The return ship should arrive separately with a minimal crew and with nearly full tanks.

The first crew should spend some time on Mars on their own, inside the Starship, and maybe in buildings it erects, or tunnels it digs.

[mulmen]

Surely the return ship would already be there before humans arrive?

[alex_young]

Definitely the tunnels. Buildings would need to be made out of lead or something. No windows.