Minecraft wiki entry for Spawn, associated text: Instead of starting at world height and searching down to bedrock at the specific X and Z location, the search begins and ends at a specific Y values determined by the type of structure. Structure spawn attempts occur only on the first spawnable block found (i.e. the highest spawnable block) within that range.
The mob picked depends on the structure: swamp huts spawn witches, ocean monuments spawn guardians, pillager outposts spawn pillagers (including patrol captains), and nether fortresses spawn skeletons, wither skeletons, blazes, and magma cubes.
The population control caps are effectively 1 higher for structure spawn attempts.