iMessage is AFAIK only really a big thing in the US.
Canada too.
I only have WhatsApp installed for when I leave the country.
I totally buy this as someone located in the US, but what is everybody else using? It can’t be WhatsApp? Is everyone sending all their connection graphdata to Meta?
Yes, and the US is by far Apple's most important handset market. The other iOS-majority countries are small markets for Apple.