It's something I've found myself do after I got into expressions, but just how common is it? Do you observe and learn?

An example:
Student wants to confirm a fact with the teacher, and as the teacher is trying to inform the student, the student creates a baseline of nods and "yeah". The student says "Yeah" a bit louder, and shows more movement in the head when the teacher pretty much repeats what the student has said - this makes the teacher want to keep saying it so that the teacher receives support-, and the student says "yeah" quieter once the student is content and barely moves their head.

I've learned what autism looks like from watching you OP.