What is this Audio and Video Track Selection and why do they appear to not work in Firefox?

[Erikcht]

Sorry, I asked a rhetorical question.
I ask this question to try to understand what Audio Track and Video Track are in your conception and to try to understand what the concept of track selection means to you?

Why is it still said today that it doesn't work? What is needed to make it work?

Is there any real test on this concept?
I ask this because what I saw in the w3schools tests and what is transcribed in html.spec.whatwg does not seem to be the same thing, or it seems that this test of what I am thinking of as an audio track does not yet exist.

Would these be Audio Track Selection?

Would one of these be Video Track Selection?
This?

Or this?