Nextcloud is free software for file hosting and a collaboration platform. Originally accessible via WebDAV, any web browser, or specialized clients, its open architecture has allowed its functionality to expand since its inception. https://fr.wikipedia.org/wiki/Nextcloud
* I use Nextcloud to synchronize (or rather copy) photos and videos from my smartphone to the server.
* I also use it to share folders publicly, with read and write permissions management (and you can easily generate a QR code to grant access).

If you want to obtain the secret key to use OnlyOffice in Nextcloud, simply connect to the Yunohost container via SSH and run the command “documentserver-jwt-status.sh”.
The key obtained must be copied into the OnlyOffice server parameter field (in the Nextcloud administration interface).

Sometimes (too often) Onlyoffice stops working with NextCloud. I don't really understand why, and I haven't found a reliable solution to keep it running smoothly.

I recommend you to have a Yunohost container apart for Nextcloud and Onlyoffice, these are two large packages, with a lot of dependencies, and it's easier to maintain with less risk of conflict if these two packages are installed apart from the other Yunohost packages on your server.