And if you are really interested. There is the LinuxONE community cloud by the marist college. If you do open source and want to add support for mainframe / s390x you can get a free tir from them.
There is a thing called LinuxONE. That’s Linux for IBM mainframes. So basically you can run anything that runs on Linux (and can be compiled on s390x) on a mainframe.
As with most other platforms there is no system & compiler developer option. I do device drivers, kernel internals, clang and gcc internals for a living. What should I select from that profession list?