if you create files directly on the OS, they will not appear directly on nextcloud, you need to re-scan the system. Nextcloud's native 'internal app' to do this is called the OCC.
you can download a plugin directly on the web interface where you'll be able to run the OCC and its commands.
in our specific usecase, we need to run from the command line as we
you can run this command:
sudo nextcloud.occ files:scan --all