I have a new coworker since last week. We are the only ones in the department. the first day, he bombarded me with questions and suggestions for improvement without any institutional knowledge why things are the way they are, rather than trying to focus on learning the core issues. I gave him short explanations as sometimes there are such long historical reasons and it can’t all be explained in a short phone call. Anyway, this strange man hasn’t responded to a single email from me since he started - I sent 4: informing him of crucial appointments (weekly calls with HQ etc.). What can I expect from this person? Will he be difficult to work with?

We’ve had so many guys like this. They move up and on so fast. Someone I started with (several of us started on the same day) 4 years ago is now an EVP. The rest of us who
do the work are in the same spot.
Does he report to you? Are you responsible for managing him? If not, then do what's reasonable and don't take too much responsibility for him. If yes, then document everything (which you should probably do anyway).
Kudos for onboarding during a pandemic. You will figure it out with him over time. Probably just initial excitement at starting a new position, giving his input to seem intelligible? At least I hope so.