All options are possible. Sometimes an expression is fully new, sometimes it's first based on a mix of ARKit blendshapes and then modified, and sometimes it's just the ARKit shapes combined.

I also used the 52 additional facial blendshapes I've made for the vtuber version's ARKit , since VRchat in general doesn't use them.

Animated by hand using blendshapes.

In case of avatars I use a root ear bone that I animate, the rest of the bones in the chain are physbones.

BEAR