Initially, I wanted to have a clean/fresh, as it were, partition (/dev/sda4). I did not count on said partition not to show up on the Grub Menu, that is until now. I had not done anything other than reboot when, while using Google Chrome Unstable crashed as I was using Manjaro Linux. When the Grub Menu came up, there was ZorinOS between Manjaro and Mageia.
Manjaro Linux (/dev/sda3) is the main OS.
I did do an update/upgrade of Manjaro whether that had anything to the change I can not say with any certainty.
I can boot into ZorinOS from the Grub Menu. I am having issues with ZorinOS which I will address at the Zorin forum here.
Maybe while Manjaro was updating, Grub was updated as well? It was not something I intentionally did. I will take small welcome circumstances when I get them.
I installed ZorinOS 9.1 on the partition where it was originally located. It was that installation and a subsequent installation of ZorinOS 10, that did not show on the Grub Menu screen. Nor did my installation of ZorinOs 11, until several hours ago?