If an effect happens after a particular step of the Invader Phase (e.g., "after the Build Step") and there are multiple Invader cards in the corresponding space, does it happen after resolving each card?

If an effect happens after a particular step of the Invader Phase (e.g., "after the Build Step") and there are multiple Invader cards in the corresponding space, does it happen after resolving each card?

No, resolve the triggered action(s) after the entire step is over, once you've finished resolving every card in that step. If the triggered action references the corresponding cards, it refers to all of those cards.
So, for example, when up against Habsburg Monarchy (Livestock Colony) Level 1+ and have both a "Jungles" card and a "Mountains+Wetlands" card in the Build Space, then (in order):
  • The Invaders build in Jungles;
  • The Invaders build in Mountains and Wetlands;
  • "Migratory Herders" triggers, gathering Towns from Sands into each Jungle, Mountain, and Wetland.