Don't forget that it is a symbiotic environment. Spotify needs to be on the Apple ecosystem as it is massive and Apple needs to have Spotify in its ecosystem because that's what the users want.
As for payments, they all follow the same scheme more or less and redistribute about 70% of the revenue to the rightholders. If the services are priced the same, it's exactly the same for artists.
The big difference is that Spotify has a permanent free tier, which admittedly makes less money short term, but it is driving acquisition of premium users faster than any other methods. The alternative being no revenue, it's just fine. Apple on the same topic doesn't care about razor thin margins on this product, or even losses. They are a hardware vendor, so as long as they still sell their devices, they will be fine.
Well, why don't Apple Music pay its "fair share" of 15%~30% to Google since it's hosted on Google Play? They are also using Google's tools and ecosystem.
Google also takes a 30% commission.
Apple doesn't have to pay because Google is so generous as to allow them to link to their own payment method.
Spotify also uses Apples infrastructure and marketing tools for its app. I dont think Ive seen any app creator up on stage at Apple events more than Spotify. Its not like Spotify will use Apples tools and ecosystem one time then stop. Its ongoing and 15% is reasonable. Apple literally changed the app store rules to accomodate Spotify and everyone knows it.
I will say that I think Spotify should be able to advertise payment outside the app.