You'd be amazed at how little the underlying ecosystem has changed over the decades. Because it basically keeps compatibility forever, software that was written in the 60's can still run on modern machines. Because of this, learning using 80's software in the mainframe world is not too bad. For example, I keep a book on MVS on my desk from the 80's for reference, and most of the time it suffices for what I need to consult.
Start by installing and running the tk4 MVS distribution. There are lots of youtube videos about it. Watch everything you can find in video about mainframes. Read a lot of IBM docs. Start by the ABC's of Z/OS Programming or some older equivalent, since you won't be running the newest system on the emulators anyway.
They probably don't share much nowadays, but look up CBT tapes for an counter example of Banks doing open source and code sharing before it was a thing.