Well, yes, but…
nextcloud forked owncloud back when there was only the php codebase.
opencloud forked owncloud ocis, which is a rewrite in go.
So while both forked “owncloud”, or “something named owncloud”, i doubt they’ll have any code in common.
Owncloud seems to be pretty much over IIRC.
The company behind it got bought be some american company in 2023, that promised that everything will “stay as open as it is” - you won’t believe what happened next ;)
Then recently many of the developers left to join OpenCloud, which seems to be a fork of owncloud, lead by a german open source veteran.
Wera for screwdrivers and wrenches. Best tools I ever had.