... ver_vs_rocky_linux/

Not exactly... Rocky is one of the clones of Red Hat... Alma Linux is another one and honestly the one I would recommend. I have nothing against Rocky Linux, but they are behind Alma in a lot of ways. Don't get hung up on the fact that Rocky is by the "orginal" developer of CentOS, because when he was, CentOS was NOT based on RedHat, it was based on CAOS Linux and the developer had left long before CentOS was based on RHEL. BTW, I have nothing against Gregory Kurtzer or Rocky Linux, it's just that Alma seems WAY ahead of the game compared to Rocky and was founded by CloudLinux (a well known cloud based hosting service that had it's own cloud based clone of RHEL already).

That said, Ubuntu Server vs Rocky/Alma is tough, and if you are trying to learn about servers for commercial use you should also consider OpenSUSE Leap (based on SUSE Enterprise Linux which has become much more common since the CentOS fiasco in enterprises servers and not just supercomputers). CentOS also is not dead, it is moving to CentOS Stream and many enterprises are actually moving to that in some instances... It would be a viable option to consider as well.

Ubuntu Server is easier to use and most Ubuntu terminal things work in Ubuntu server, but it is used much less in enterprise environments, one of the CentOS clones, CentOS Stream, or OpenSUSE would be much better as a learning model.