Comment by joshuapowell

Great question. I work in the mainframe division of a Fortune 100 company.

The suggestions above are great if you're interested in learning about the technologies used on z/OS.

A vote for zeeframe's recommendation of the IBM Z Xplore course. Great stuff.

If you're looking for more reading material, I'd suggest the O'Reilly book "Modern Mainframe Development" book that covers a range of topics. (https://learning.oreilly.com/library/view/modern-mainframe-d...)

If you're looking for an on-the-job training approach that would enable a career change into the mainframe, Broadcom offers something called the Vitality program (https://mainframe.broadcom.com/beyondcode/vitality_program) that runs every year in North America, Europe, and Australia. It's a program that would equip you with serious credentials that could help you land one of those System Programmer jobs or similar.