Probably a mix of Z systems, that stuff goes back 20-odd years, and even then older code can still run on new Z systems which is something IBM brags about.
Mainframes aren’t old they’re just niche technology, and that includes enterprise Java software.

Uh, Java is specifically supported by IBM in the Power and Z ISA, and they have both their own distribution, and guides for writing Java programs for mainframes in particular.

This shouldn’t be a surprise, because after Cobol, Java is the most enterprise language that has ever enterprised.

No thanks, people hop from centralized platform to centralized platform thinking things will be different.

Consolidated power, especially when fully captured by market forces, will always enshitify, the only out is federation.