That confirms that the process is not running. Try running sudo systemctl start mongod. After a couple of seconds rerun systemctl status mongod and it should show that it’s running. If not you can show the error log and we can help troubleshoot more.
It seems you created a solution to a problem you didn’t have. Based on the fact that you had mongod as a service, the installer would have already created a data directory and a user with limited permissions to run the process. All you needed to do was run sudo systemctl start mongod (there are ways to get around using sudo to start a process, but every time I type sudo it makes me think about what I’m doing and why I’m needing elevated privileges) to get the service up and running.
There are limited cases where you should run a service using sudo, and mongod is not one of them. Running with sudo introduces potential security risks.
As for creating /data/db, that is where MongoDB put the data files when it was first created and for several versions afterwards, but I believe the new location on Ubuntu is something like /var/lib/mongo which is more in line with standards and best practices.