So how does a computer cause “undefined” behavior to occur? Does it invoke the “undefined” opcode? Of course not. A computer executing a program that has entered UB will do something specific but not specified in the standard.

I don’t think you are getting that Undefined is a concept in the language specification domain, that is not literally implemented. Consequently, you don’t even see yet what my question is about. When I think of a way to make it clearer, I will follow up.