There's a bunch of quite good speculation like this in Plan A

https://ai-2040.com/supplements/economics-of-plan-a

And yes, moved into skyscrapers:

"This implies that the price of housing will become dominated by the price of the land (as opposed to the cost of building housing), so dense housing (e.g. skyscrapers) will become relatively much cheaper than sparse housing"

There's a later bit that talks about keeping earth as a reserve too.