When you add a domain in Cloudron, you are just adding a domain for installing apps under that domain. The only DNS configuration it will do is to fix up the SPF record so that Cloudron can send mail on behalf of that domain (it adds itself to an existing SPF record). So, if you install say gitlab, gitlab can send mail to you.
When you go to Email -> Enable Incoming mail, at that point, Cloudron will start receiving incoming email for that domain. It will setup DKIM/MX/DMARC records as well at this point.
You can send/receive email without any of the webmail apps (rainloop, roundcube) installed. The webmail apps are really just frontends just like thunderbird or your android/ios mail client. On Cloudron, we pre-configure these apps to work only with Cloudron email. We could make a package that does not do this but that use case is quite rare because most email providers have their own web ui anyway.
I think @necrevistonnezr just wants to add a domain into rainloop. This is the equivalent of adding an account in thunderbird. It has no effect on Cloudron mail server whatsoever.