For the US it's the accent located in the population center of the country, of course. A little south of Kansas city on the Missouri side of the river.

I would say that it's a nebulous and organic process that settles into particular pronunciations. One thing a lot of people don't realize is that the British accent is actually considerably more divergent from the one that was common to America and England in 1600 (I can't seem to find the source right now, but will edit if/when I do). I'm not saying that it's right, just that it happens and that there are valid reasons for it happening.

We do this by consensus, instinctively choosing whatever is most typical for the powerful, or perhaps a slightly exaggerated version of that.

So: doctors, lawyers, politicians, rich people, random TV personalities, CEOs, generals...

So the same accent as Malcolm Turnbull, Jacinta Ardern, The Queen, Donald Trump, Leo Varadkar, my Polish GP, my Italian dentist, Elon Musk, Alan Sugar and Duncan Bannatyne.

I'm not sure that works.