A mob can spawn wherever any one of its conditions holds in full.
Loading the vanilla spawn rules...
All 60 spawn rules in Bedrock 1.26.52 are here, from armadillos to zombified piglins, and each opens with the exact conditions the game uses. A rule is a list of conditions, and a mob can spawn wherever any one of them holds in full, so a zombie with two conditions spawns in either place. The editor covers where the mob spawns, the light, height, weight, group size, difficulty and biomes; less common settings, like a mob that can turn into a variant when it spawns, stay exactly as Bedrock has them.
Bedrock doesn't check spawn rules when a world loads, so a broken one fails silently. That's why the editor only writes the shapes the vanilla rules use, and an edited rule was tried in a real world: with husks moved to every monster biome, a Bedrock Dedicated Server spawned 7 of them around a simulated player in a minute in plains, against none without the add-on.
Spawn rules pick biomes by tag rather than by name. Each Bedrock biome carries a handful of tags, 98 in all, from broad ones like overworld and monster to narrow ones like mooshroom_island. Ticking a tag in the editor shows which biomes it covers before you use it. For which mobs spawn where today on both editions, see the biome browser.
Open its rule, press Change biomes and tick the monster tag for hostile mobs or animal for passive ones; 75 vanilla biomes carry the monster tag. Husks, for example, only spawn in the 3 desert biomes, and with monster ticked they spawn on the surface in all 75.
When the game picks a mob to spawn at a spot, every mob whose rule allows that spot is in the draw, and weight is its share. A vanilla zombie in most biomes has 100 and a husk in the desert 240, which is why deserts are full of husks. Raising a weight makes the mob more common without changing where it can spawn.
Monsters spawn all the time in the dark, so a change to a hostile mob shows within a minute. Animals mostly spawn when new chunks generate, so travel somewhere you haven't been yet to see a change to cows or sheep. Mobs also count toward a cap for their kind (monster, animal, water animal and so on), so a full cap holds new spawns back.
The numbers are light levels from 0 to 15, the same scale as Java. Most vanilla monsters on Bedrock spawn at 7 or darker, while Java monsters need complete darkness; the editor shows each mob's own range, so you can match either edition.
