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
① Oh, because though Sam is thinking--this is almost a standard expression.
② Hmm, because Jan is thinking about the reason.
③ Yeah, because Sam is confirming Jan's suspicion or guess.
④ Uh, because Sam is thinking and searching for the right effect or words.