File Question

Which file determines what buildings a settlement starts out with?

descr_strat.txt in the data/world/maps/campaign/imperial_campaign folder.

Foot

But some buildings are added by script too, even at the start.

How about the Governor buildings? (not the MIC/Govt Type ones)

Those are all in descr_strat.txt for sure.

And if you change the palace buildings, make sure you change both the governor building and the size of the city to the correlative city size or strange things will happen.

I'm actually looking into how the culture type is determined for the governor buildings

Faction culture. For those there at startup.