I originally got into mainframe tech through IBM's annual Master the Mainframe program [1] which got me limited access real mainframe on some college's campus and IBMs manuals.

The old -- and likely not so bad -- advice if you'd like to learn how to use a Mainframe is: Get access to a one, get a copy of latest Red Book [2], and start playing around :)

[1]: https://www.ibm.com/it-infrastructure/z/education/zxplore (this isn't the program I went through, but my old bookmark now redirects here)

[2]: https://www.redbooks.ibm.com/abstracts/sg248852.html (IBMs mainframe Technical Guides traditionally have a red cover)