One of my first projects was to interface with an IMS system using Python of all things, there was only one guy that really knew the system and I had to sit with them for hours to even begin wrapping my head around how a hierarchical DBMS worked, how space is managed, etc. I remember the first comment he made was that the database was created for use as an inventory system for the Apollo missions. The DB I had to work with was created in the 80's. This project was in 2021, so it's still out there, supporting critical infrastructure.