Almost all large banks are running mainframes.
Banks, insurance companies, payroll providers, airlines reservations, financial management concerns, telecommunications services, subscription billing vendors, governments, the military ...
Yep. Mainframes are dinosaurs. /s
Many big corporations with global operations run multiple “mainframes”.
I did some work at the 3rd largest electronics distributor in the world. Overnight their mainframes - in USA, Asia and Europe - are keeping all their global databases number-crunched, up-to-date and backed up; and during the day the same equipment - world wide - looks like a bunch of “networks” comprising their entire global Intranet, simultaneously running some old “green-screen” and up-to-date “X86” file-server and client-server apps.
IBM has reinvented what a “mainframe” can do, so that it can do just about anything through virtualization of the supposed non-mainframe world, on the mainframe.
I know one outfit that runs a small “server farm” on a mainframe.