Birch forest hills are biomes that occur when a hilly and mountainous area is covered with birch trees. When the birch forest is generated in a hilly biome, the trees are taller than usual. Other than that, the birch forest hills are pretty similar to regular birch forests.
Birch forest hills are a variant of regular birch forests that occur on hills and mountains. They don't have a lot of wildlife spawns but the monster spawn rates are unaffected. This biome is somewhat rare since birch forests usually generate mostly on plains.
A. Yes, birch forest hills are rare biomes since birch forests are usually found in plains.