We have some info & tutorials on the wiki http://wiki.polycount.com/wiki/ChannelPacking http://wiki.polycount.com/wiki/MultiTexture Some examples from Unreal https://docs.unrealengine.com/5.0/en-US/using-texture-masks-in-unreal-engine/…
Regarding the hair, the polycount wiki might help you out: http://wiki.polycount.com/HairTechnique Especially this one: http://wiki.polycount.com/HairTechnique?action=AttachFile&do=view&target=squirrelyjones_fur.pdf