In France (and basically in all European countries, maybe except Germany, where all privacy things are very intense so maybe they use something else), WhatsApp is the go-to app.

Not only do most people use it, but you have services that suggest WhatsApp as one of the contact channels. This is not because they love WhatsApp for some reason - rather because people use it (and they want to make it easier to contact them).

You talk about "WhatsApp groups for X", never heard about "Signal groups for X". In rare instances, this would be Telegram (but usually specialized groups)