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.
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.