Content that’s consumed on the device to be more precise.
You can sell domains and hosting etc. using alternative payment methods. For example I use namecheap’s App to buy domains and I would use CC to do so.
So why not ads then? Well Apple doesn't have insight into the ad spending, and they are not a middle man to the cash transfer. They could try to force themselves into that position, say "if you want to have ads in your app, you have to use our SDK for that". But if they tried such a stunt, then Google and Facebook, would become very powerful very motivated enemies.
https://www.theverge.com/2020/4/1/21203294/amazon-prime-vide...
Interestingly they also allow you to deposit with Apple Pay (along with many other payment processors, which the user can choose).
My guess would be that the rule is that there's no Apple cut if you're selling a physical thing and there's a digital attachment. But Apple probably reserves the right to disregard sham physical transactions, where the physical item is de minimis in value compared to the digital item.