Spawning System

"More Mobs" has its own spawning system. Functions are used in a variety of ways to spawn mobs in the data pack. Under certain rules, vanilla or "More Mobs" creatures will be replacing existing mobs in various biomes, dimensions and structures, partially or fully. The spawning system has evolved with introductions of new data pack functionality.

Random mob spawning
Similar to iNkoR's other data pack ("The Aether"), it utilizes random entity rotation with which most mobs in Minecraft spawn. You can check this out for yourself if you spawn a row of mobs with a spawn egg. There is a range of rotation values allowing to choose the commonality of "More Mobs" creatures.

Mob replacement
In some cases, mobs in the pack are replaced with other mobs. Certain mobs will be completely replaced with other depending on the structure/biome/dimension in which they have spawned. Vanilla mobs will often be replaced with "More Mobs" creatures, or other vanilla mobs, and vice versa.

Raids
The raid event has its own set of spawning rules.

Pre-3C
First, the player's location is determined with advancements. Then, some mobs are selected (sometimes based on what blocks they are standing). This method is used to spawn in mobs randomly or to replace mobs completely.