Do you want Unbound to query upstream DNS over HTTPS servers or do you want unbound to answer to DoH queries?
For the former, unbound cannot query upstream DoH servers, only DoT and DNS at the moment. An issue is still open for setting DoH as upstream. A solution is to use cloudflared or dnsproxy to proxy DNS queries to DoH upstream servers.
As for the latter, unbound can be set to answer to DoH queries.