I’m not sure how nextcloud handles it, but as long as you can resolve the domain then you can put whatever you want.
You usually purchase a domain so it appears in the internet with the major DNS’, but if you only have the site in your internal network then you can put whatever you want as long as you update your internal DNS.
Usually you can do this by manually updating the hosts file in your machines.
But a better way is to have something like PiHole, in which you can set your local DNS to resolve to your own IP.
After that the only annoyance are the SSL certificates which will be selfsigned since browsers show a warning but some services don’t have a way to work with them.