Have spent a couple years about 20 years ago dealing with a COBOL applications, I can tell you. Pick up COBOL (GNUCobol is fine) and a book. It isn't that hard of a language to pick up.

Then, learn about mainframes since most of the COBOL in the wild is running on z/OS or the like. IBM use to offer a free Mainframe course. You would also want to learn JCL, REXX, and a few other mainframe related tools.

The apps are no different then most things now (inventory, accounting, etc.) is is the legacy processes that will most people that biggest problems.

And checkout a YouTube Channel by moshix. Lots of great mainframe info there.

That would get you started on the path.