This is mostly apocryphal in the US as well, according to my school-age kids. They’ve been in many peer text chains over the years in several SoCal schools, and they’re invariably multi-platform (i.e. good ol’ green SMS/MMS).
Not really, WhatsApp is used in Europe, Middle East and India. Japan, Taiwan, Thailand, South Korea are on Line. China Weibo I think. Even in the West the market is fragmented with Signal, Telegram, Facebook Messenger, Instagram, Discord, etc.
The important commonality here being “not on iMessage”.
„just“ Europe, Middle East, India and South America (btw)