Teach me about biryani

I was watching this video. The auto-translated subtitles are not great, but I followed along a bit.

We tried 15 types of Biryani

It made me realise that in the UK I have access to a very limited selection of biryani. From a supermarket it will look like this: https://www.iceland.co.uk/p/iceland-chicken-biryani-375g/87458.html. I'm missing so much knowledge about an enormous region that covers over a billion people.

I'd be really interested to hear about biryani, especially regional variations with different ingredients. What things are essential and often missed? What makes a biryani great?

I'd also love to hear more about delivery - those "handi" ceramic dum cooked to order pots look amazing. There's another video here of an "unboxing" - https://www.youtube.com/shorts/Q5OA4XiGl34 , and the makers have a video here too: https://www.youtube.com/watch?v=H6nE1Nla3u0