This isn’t really true. The old terminal monitors are essentially dumb CRTs and the the museum pieces are just displaying the output from a regular PC running the modern app converted to an appropriate signal + BNC connectors so you see something and not just a black screen.

The old terminal hardware communicated over dedicated serial lines and worked in conjunction with many other “server-side” pieces of software that no longer exist, so it would be impossible to make one function as it did back in the day.