What’s the best solution for creating and managing email addresses for each domain? Ideally, I’d like to set up addresses like hi@domainA.com, hi@domainB.com, etc., and have a single, easy-to-use inbox where I can send and receive emails for all of them in one place.
Similarly on the SMTP: You can send directly yourself or use separate relays for individual domains.
Then I guess it's up to you if you want to keep inboxes compartmentantalized and duplicated (internal forwarding) or suck them all into one place directly (aliasing).
Reasonable mail clients will allow you to add multiple sender aliases and map them when you reply.
You can set up e.g. dovecot and postfix yourself or use one of the many appliance projects out there (docker-mailserver, mailinabox, mailu, mailcow, I'm sure there are others). Or if self-hosting is not an option, providers should slso support adding domains to your account (usually for a pretty penny).
That can be the end result if you go with a webmail client like Roundcube or SoGo, yes.
If you go with a managed provider, as mentioned, I think they should all have some options for that in their paid plans and you'd have to consult their docs on it.