You can finds lots of books and online resources on COBOL programming or JCL. There are also encyclopedia sized publications called IBM Redbooks (available in

You can finds lots of books and online resources on COBOL programming or JCL. There are also encyclopedia sized publications called IBM Redbooks (available in pdf if you Google for them) which provide tons of detail on hardware and OS level concepts you need background understanding of. As for how you'd actually play around with writing usable software.. you'd need access to a mainframe. Basically, the insane complexity of mainframes and the walled garden nature of the ecosystem makes it extremely difficult to teach or to learn. I worked on the z/OS network stack for a few years (tech support and later some actual development) and it was the most complex thing I've ever tried to learn in my career.