Mobs that only spawn in certain areas do not spawn. For example, guardians do not spawn in Monuments, and Wither Skeletons/Blazes do not spawn in Nether Forts. In addition, no mobs spawn in the Nether or End. Mobs that generate as world spawns (i.e. The Ender Dragon, Shulkers, and Elder Guardians) still spawn as normal. Regular overworld mobs spawn as normal as well.