Telegram? In my experience, European countries seem to use it alot
Mostly Eastern European countries. Some fringe far right groups in Western Europe use Telegram because of a percieved higher level of privacy. If WhatsApp went down for any considerable length of time, countries like Spain would fall apart. I do recall an outage of several hours a couple years back that contributed to a huge spike in Telegram and Signal installs.
Then, sadly it went back up and everyone just went back to WhatsApp.
I'm not in either of those groups you described, thankfully not in the latter! Maybe it's just due to me being a techy.
Nonetheless, the move you described, even if short lived, shows that the effects aren't as dire as one may imagine.
2bn monthly active users is rather a bit more than "Europeans".
Having a group chat with a mix of iOS and Android is painful on anything other than WhatsApp.