Exchange Server, how to organize the circulation of external mail for internal mailboxes more competently?

Question

Mail server

There is a great desire to put an Exchange server 2013 or 2016 at work.
Everything is there for this, except for the experience of working with this "monster", or rather, it is prefabricated and there is not enough of it :)
The server is planned as a purely internal one, on a local domain.
There is a certain host the provider at which mail of the enterprise turns
Question in the following. How to assign several users of the local domain with local mailboxes to receive mail from certain external mailboxes, and send it to the Internet from external mailboxes, respectively.
I understand that this is all implemented through “Mail Flow” and Accepted Domains”
And then? Through connectors? Or through some kind of politics? Address change?
If anyone has links on this topic, please provide, or briefly say which steppe to dig