Huh?

From Twitter:

Sam: Hi Jan.
Jan: Hello, Sam.
Sam: How’ve you been?
Jan: All right. And you?
Sam: ①, not bad, but it’s getting hard.
Jan: ②, you mean social distancing?
Sam: ③, not meeting people…. ④, it’s lonely.

Yeah
Hmm
Uh
Oh

Answers
Oh, because though Sam is thinking–this is almost a standard expression.
Hmm, because Jan is thinking about the reason.
Yeah, because Sam is confirming Jan’s suspicion or guess.
Uh, because Sam is thinking and searching for the right effect or words.