For example, I live in Thailand and the (Japan-based) messaging app LINE could probably reach almost everyone with a smartphone.<p>Seems like in most Western countries it would probably take 3 or 4 apps? What are they where you live?
One, in Denmark, we have a system called MitID (basically SSO login for public services (and private services can use it too). Even though there are other options (hardware totp dongle and such), most people have the app installed - I would guess easily over 90% (over the age of 13).