Looks like the spec refers to the thing-that-has-the-thread as an "agent". https://tc39.es/ecma262/#sec-agents
I don't know the details about how any implementation of a javascript execution environment allows for the creation of new agents.
I don't know the details about how any implementation of a javascript execution environment allows for the creation of new agents.
It's like a pedantic technical behind the scenes point I think, just trying to learn "what's true"