I wanted to make essential yeast, but I could not make them in the mushroom and dark forest biomes. The config says that this yeast can be obtained in the MAGICAL biome, but I don’t understand what kind of biome it is.

There used to be a utility in Growthcraft for dumping the biome names and their types, so you could review that file to find it.

But it seems it wasn't ported, maybe @GrowthcraftCE/growthcraft-1-12-2-devs could help you with that

Or possibly port the biome dumping tool to 1.12

https://github.com/GrowthcraftCE/Growthcraft-1.7/blob/049f0929f85a25b9eda9ba56555954155a1d2cfa/src/java/growthcraft/core/util/GameRegistryDumper.java#L198-L218

Technical stuff aside, if you have thaumcraft then the biomes it added should all be magical.

It's been a long time since I looked into it so I could be wrong.

It's for use in mods that have biomes that use that Biome Dictionary definition. BoP has a few examples.