Do they offer many services for *OS apps or are they mostly independent and only service their own first party apps?

Theoretically they offer quite a few; in practice, I think nowhere or almost nowhere I've ever worked has gone for those options (except my own self-published games), instead preferring a custom solution that can be shared with web and android.

iCloud is the backend for basically any settings/file sync for normal apps.

But it is mostly used for periodic backup/sync. So you aren't going to really notice unless it is a very long outage.

No website uses Apple

A sync issue in drive would barely be noticed.

Apple Pay, with 65M users, can rely on external content from Apple.

https://developer.apple.com/documentation/applepayontheweb/d...

That's not a website

You'd probably be affected if you used Sign in with Apple, same as if you used third party sign in with Google, Facebook, Microsoft, or others and their authentication was down.

Sure, I don't use/have it though.

Because the consumer market is way less demanding than businesses. Hence why B2B and B2C are categorized separately

Half the world? Not even remotely close. Not for MacBooks or iPhones or anything else. They're a niche player.

Apple market share is about 30%

For mobile - For desktop computers, below 10% even. Given that only 73% of world population is estimated to have internet access in total, that makes it just a small fraction of world population overall

As a portion of Internet users who regularly pay for and depend on hosted services, I bet that percentage is much, much higher.

US marketshare is higher.