Instead of rewriting everything, here a translated quote from the german wikipedia-page:
"Nextcloud is free software for storing data (e.g. files, calendar, contacts, etc.) on a server.
The user can access the data both via a web interface and with client applications (smartphone and desktop).
Server and clients can synchronize themselves. Nextcloud enables the user to access a central and consistent database
from many end devices and optionally share it with other users.
In addition to data storage, Nextcloud offers functionalities for video conferences and various office applications
via the web interface. "

(Stand: 14. Dezember 2022, Lizenz: CC-SA
It scales from a Raspberry Pi with 2 users to globally distributed installations with millions of users.
I can help you or your local IT-person setup your private cloud in your small office.
For more information, see officical nextcloud webpage.

If you're interested or have more questions about a private cloud using nextcloud, do not hesitate to contact me for more information