This is probably a dumb question, but what language an early COBOL compiler was written? Is that assembly? How the language runtime was connected to the I/O? Wa

This is probably a dumb question, but what language an early COBOL compiler was written? Is that assembly? How the language runtime was connected to the I/O? Was there even an operating system? COBOL and mainframe seems so arcane to me that I'm not sure where to start understanding the whole stack.