In 1.17 and below, badlands are meant to place gold below the surface and mountains were meant to place emerald and infested stone below the surface. That used to work pretty well, although 1.18 introduced 3D biomes which means another random biome (dripstone/lush cave, or another biome) might generate below which causes the areas where the gold/emerald was meant to be to not contain gold/emeralds.

As you can see in the screenshot, it is badlands above but because it is technically a forest where the player is at, no excess gold is generated. This is rather unintuitive, as there is no indication other than the F3 debug screen that no gold will generate.

Lush caves for example generates azalea trees even if it's technically outside of the biome. Badlands and mountains deserve the same treatment.