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