Structures can spawn on the surface in any biome as long as someplace at some y level is their native biome.

as shown below.

Reproducable by:

Version [1.18 Experimental 1]

Seed [1553755667127605030]

Running [/execute in minecraft:overworld run tp @s -20095.50 100.01 1856.50 -590.59 75.15]

at these coords on the surface level there is a mansion but no dark oak biome. at y-36 there is.

causes mansion to spawn on surface not in dark oak.