Where can I find the spawn biomes & spawn rates of vanilla mobs?

Look for usages of the

SpawnListEntry

constructor using your IDE.

I can't find what biomes they're spawning in

If you know it, tell me what line, what class

I can't find any biomes set for the vanilla mobs

I don't understand what your issue is.

 

Minecraft uses

SpawnListEntry

instances to control which entities can spawn in each biome. If you look at where this class is instantiated, you'll see which biomes allow which entities and what rates they use.

Here:

Then what is the correct search tool, and the biome class match was the same as the stuff under there

You're a fucking idiot.  The very first entry you opened up was a wrapper call.  You never even bothered to trace the calls for that method.

Following that, you ignored every other result just because it came from main()

There is no such thing as find references in Eclipse 4.6.0.20160203-1802

Well, thanks