The custom stuff comes down to OS-related calls and understanding how DASD is used.
It’s pretty easy to program, outside of the OS/DASD custom stuff.
Started with Burroughs in 1981, learned a little Cobol while there doing mainframe support.
Recently I saw job listing in Detroit, that wanted someone with Burroughs/Unisys A-Series Cobol and DMS II experience.
Unisys was what Burroughs became after they merged with Sperry-Univac....I supported Cobol and DMS II which was their Database system when I left Burrougs in the early 1990s......
I am still coding COBOl. Recently learned SSIS. Not sure about the.net stuff though.