Once you are on a mainframe, the most important command is: =3.4. This command is issued on the command line of the so-called ISPF screens.
If you are in this screen, simply issue =3.4 and you are in the screen that gives you access to all files. This is the stepping stone into the work that you might create: it gives you access to output, input, jobs and everything else that is important in your mainframe life.
There is a second command that is of equal importance: the PF3 command (for some reason, the IBM mainframe boys and girls use PF3 for the F3 function key). This means: go one step back. Repeatedly tapping the F3 key leads to the exit: the end of a session. I noticed that the PF3 command is also labeled as „End“. This means that if you read : „enter end command to return to the primary option menu“ should be interpreted as press F3 to return. Do do tap on the end key: it does not give you the desired result. No use the F3 key. Hum, a fact of life.
I would place a bet that you are able to do a simple session on a mainframe with just these two commands: the =3.4 and the PF3 command. If I was to write a quick guide for a mainframe, I would start with these two commands.