System Z - as I understand it, this is descendant from IBMs earliest computers like System 360. Typically apps are written in COBOL, PL1, tied together with JCL. “Newer” apps use DB2, but there are older non-relational databases as well.
System P - an IBM alternative server architecture based on the Power chipset. I think these mostly run UNIX, but I’ve never been hands on.
AS400 - technically an IBM minicomputer, but similarly esoteric with a bespoke OS and some nifty functionality for business apps (very SQL centric).
HP Tandem - traditionally used in high availability transaction processing applications.
Others? I think most of the other surviving old stuff is basically UNIX
Typically people are thinking of System Z. I think both the inertia and scalable ACID explanations are relevant. That said, I’m not sure how DB2 on System Z compares to say Oracle RAC.