In 1989, I supported as systems programmer Unisys mainframe at State Rail Authority of NSW, Sydney. I am pretty sure it ran OS 2200 operating system not Unix. It had disk packs and tape drives in a special airconditioned control room with raised floor for cabling. This was a heavy duty systems programming role.
In 1981-1987, I had studied operating systems and systems programming in my BSc at University of Queensland majoring in computer science.
This was before Linux was developed so operating systems were closed source back then. Now with Linux and FreeBSD, some operating systems are open-source so can be adapted much easier than back then when it was very hard to fix problems in the operating system due to relying on the vendor to supply patches and updates. Now Linux and FreeBSD systems programmers can write their own device drivers for new peripherals or devices or disk drives and hack the kernel to get better performance or improve security but back then that was difficult or impossible. The IT industry has evolved a lot in the past 30 years!