How to sync/single sign on nextcloud users with local host server credentials

Hello,
I have a nextcloud on a ubuntu server for my lab, and currently people use an username/password for the server (jupyter, Rstudio,…) and one for nextcloud.

Which is the simplest way to let Nextcloud to use the user accounts on the server (in my case under group “users”) rather than duplicate the accounts ? (LDAP seems overkilling)

You can authenticate against other backends:
https://apps.nextcloud.com/apps/user_external