Technically Nextcloud is a PHP application behind nginx or Apache with PHP-FPM. Add a database (MariaDB, MySQL or PostgreSQL), Redis for file locking and caching, a cron job for background tasks, and the data storage: local, NFS or an S3 bucket.
Much of the administration happens through the occ command-line tool: upgrades, maintenance mode, file scans and app management. Settings live in config/config.php, errors in nextcloud.log in the data directory.
Most operational problems do not come from Nextcloud itself but from its surroundings: upload limits and timeouts in the proxy, too few PHP-FPM workers, missing Redis locking, a cron job that does not run, or wrong trusted_proxies and overwriteprotocol values.
Example
sudo -u www-data php occ status