Bit confusing to me when Apple charge on iOS apps and don’t

Considering the current investigations into the iOS App Store rules and policies, I do find it somewhat confusing when Apple would charge and not.

In their “App Store – Principals and Practices” document (https://www.apple.com/ios/app-store/principles-practices/) they give examples of types of app categories and some example named apps, but to me even some of these don’t always make-sense.

FREE: an example app is “Wikipedia”, which is well-known for doing regular donation-drives. When such a drive occurs within the iOS app, can you choose to pay via any payment method, or only via Apple Pay, and if the latter do they take a 30% cut of the donation? (Similarly, how does it work for charity apps where you can make donations?)

FREE WITH ADVERTISING: it appears from the wording Apple make no revenue on these apps at-all. But don’t apps on iOS have to use Apple’s ad platform to serve ads, and so they would make revenue that way from them, just like Google does with adverts in many Android apps?

READER: confusing. Apple say that developers “receive all of the revenue they generate from bringing the customer to their app” which I guess is shorthand for “if they sign-up outside the app, we’ll let them sign in” but isn’t part of Spotify’s argument they don’t actually allow this for them, yet for things like Netflix and Amazon Prime they do? I’m also not sure how you “read” music or “read” a movie (unless you get a songsheet or transcript, or use subtitles 😉 ).

FREE WITH SUBSCRIPTION: worth noting the wording says “after the first year, the developer earns 85% for all successive years THAT THE USER REMAINS A SUBSCRIBER, and Apple collects a 15% commission”. So if someone ends their subscription, then starts it up again later-on, would they be classed as a new subscription so it goes-back to 30% again for one year?

CROSS PLATFORM: these are basically the apps that Apple just let you sign into with an external account and don’t force the developer to promote signing-up inside the app and only showing Apple Pay as the only payment option. But this category in itself is confusing: why are Word and Dropbox given as examples of apps which are cross-platform, yet Spotify, Skype and Netflix aren’t? Netflix is available on Android, Windows, macOS, games-consoles and some Smart TVs; Spotify the same, but it also offers a web-version; Skype also has an official Linux client. So, how are any of these not “cross-platform” when apparently Microsoft Word is (despite not being on any games-console, Linux (as an official-port, anyway) or any Smart TV)?