iMessage is AFAIK only really a big thing in the US.
Yes, and the US is by far Apple's most important handset market. The other iOS-majority countries are small markets for Apple.
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?
Canada too.
I only have WhatsApp installed for when I leave the country.