It's 30% for "in-app purchases" which is a specific API which allows one-click payment for one-time purchases or subscriptions ONLY FOR digital goods, and the payment goes through the iTunes account (eg: credit card).
If you buy a real world product or service, you need to use a third party payment service like Stripe (or Apple Pay, if you want), there is a 0% commission fee from Apple. This is why Amazon can distribute an app on iOS that allows to buy whatever you want without Apple getting a cut; well, anything but digital goods, like Kindle books, which in fact can't be bought from Amazon iOS app.
And most importantly, it's not 30% for Apple Music.
What do you mean it isn't? Apple Store could trivially take a 30% cut of the Apple Music revenue, and the only thing that would change would be the distribution of revenue between Apple Music and Apple Store in Apple's revenue reports. The total revenue would be the same.
Exactly, but Apple can run their streaming service as a loss leader because they still get to book the 30% revenue on the store side. Spotify can't recoup that 30%.
This effectively gives Apple an extra 30% margin for their streaming music service.
It seems that 30% is for features that are delivery in app, since Uber and Deliveroo are delivered outside app they have a lower cut. It's just commission cut rules by Apple.
Regarding Apple Music I'm not sure what that supposed to mean. I'm pretty sure that Spotify doesn't charge the same for "Spotify Premium" and "McDonalds" ads.
As they mention on the website. That's not true. It's not 30% for Uber or Deliveroo. And most importantly, it's not 30% for Apple Music.