There is a lot ground to cover, I honestly don't have all the specifics. To me there's a lot of gray area, but it comes down to what does the song require. To simplify things, it makes sense for a choir to use the 4 major ranges, it makes writing the composition easier at least.

Those videos are interesting. The main showcase seemed to be how when hitting those notes called out, how effortless was it. I'm a tenor, yet I can still carry notes out of range. The trade-off is that I lose volume or stability. Or like how my low note might be a baritone's high note. It's the same note, but who's more comfortable with it.

Keep trying to figure it out though, it's a good rabbit hole to get into. Cheers~