All command line, and totally foreign on a level that makes switching from Windows to Linux look trivial by comparison.

(which coincidentally, is why I want to explore it)

Here's the official IBM "for dummies" book: https://www.redbooks.ibm.com/redbooks/pdfs/sg246366.pdf

It uses the 3270 terminal (xterm has a 3270 mode I've used), interface is predominantly 'ispf', it's essentially menu driven; you can browse/search the filesystem, edit files, submit jobs (everything is a job ie jcl ie job control language; compile, run programs etc) browse job spools (stdout from a job).

Google will show you images of ispf, browsing the jcl manuals will give you nightmares.

My first job was writing JCL decks - it's not that bad.

It is not terrifying per se, just ... very different.

I have only had brief contact with JCL, but I remember having to figure out how to get a command line into JCL that was more than 80 characters in length. It took me two days to figure that out, and none of the old-timers in the team had ever had to do that. When I did find the solution, it was surprisingly simple, but finding it took me a lot of time.