Nextcloud will not pick up files if they are added directly in the data directory of the user on the server. To make it rescan, open a Web terminal and run the following command:
sudo -u www-data php -f /app/code/occ files:scan <username>
To rescan external storage, use the --path parameter.
sudo -u www-data php -f /app/code/occ files:scan <username> --path=/<username>/files/externaltest