> Nesting the elements is a truly hideous choice. The summary is part of the details?? I thought they were opposites.

It gives them a semantic connection. Last I checked, HTML isn't really based on giving special meaning to combinations of sibling tags. A summary is part of the thing that conceptually requires detailing.

> If you find 1000 websites using a toggle I doubt there is one using an ugly arrow like that.

I think the default looks fine. But TFA clearly explains right there that it can be styled. (Specifically, by styling ::before on the summary tag.)

> The default styling gives no clue about it being clickable?

You asked what the arrow is, and then asked about the lack of indication that the summary header is clickable. The arrow is exactly that indication.

> Maybe I'm dense but I also want my url to reflect the state of the page.

If you scroll, should the fragment automatically update as you scroll past anchors? I think I'd find that quite annoying.