It's a fun experience to install MVS 3.8j (apparently 40 years old by now), using the Hyperion fork, to get a taste of how utterly different mainframe computing was (and still is).

If you've never dealt with mainframes, which is extremely likely after all, you will be massively taken aback about how different everything is. A lot of the most fundamental terms you are used to, like "file" or "directory", simply don't exist. Some have equivalents with other names, but even if they do they're often so different that you barely recognize them at all... It's like finding a computer on an alien planet.

Interestingly, the later "midrange" AS/400 (or maybe also its predecessor System/36 which I don't know about) is also completely different from mainstream computing, but in something that I'd describe as the opposite way of how MVS is different.

While MVS lets you feel the bare metal at every turn (making such simple things as using the closest thing to a file, a "dataset", sometimes rather bizarre), AS/400 does the opposite and abstracts everything away. Going as far as having a 128bit unified address space, which addresses both RAM and hard disk storage and everything in between...