I'm guessing these operations are mostly people transferring or buying/selling tokens on the blockchain. Would that be correct?

I can't answer that question as I have never analyzed the HE transactions.

Something happened in 2021 that caused a downward trend in daily transactions. What was it, and what can we do about it?

Splinterlands was one of the main drivers of transaction growth. I guess that after the initial hype, players got bored and unfortunately Splinterlands didn't find a second wind.

What kind of HBD interest rewards are people earning by account level?

I'm not sure such an analysis would be relevant, especially because several people have created a separate (low-level) account for HBD savings.