Question How Are Humanoid Animations Created?

I have humanoid assets with humanoid avatars. I want to create my own animations for these characters. The issue is that the Unity Animation Window only allows for one to create generic animations. I saw a workaround posted on the forum suggesting that you can animate the character using the "animator" properties rather than rotating transforms. This works but it still doesn't solve the whole generic to humanoid mismatch thing. The character falls halfway through the floor as it isn't bound to the right avatar.

There must be some way to do this. How are all of the humanoid assets on the store animated? I am proficient in C#, so if there is an API or something I am missing please point me in the right direction. I would prefer to use the recorder as it seems to be the quickest way to get this done.

Thanks in advance!