I think it may depend on where you are. Back when Whatsapp went belly up, myself, my entire family, and every other person I know IRL switched over to signal within a week, so I think it may be more popular than you expect, though still not as popular as Telegram as you noted.
Signal is nowhere close to being popular.
Telegram has almost 1 billion users, which is why governments are unhappy with it. Signal is a niche toy for a very small number of people.
I think it may depend on where you are. Back when Whatsapp went belly up, myself, my entire family, and every other person I know IRL switched over to signal within a week, so I think it may be more popular than you expect, though still not as popular as Telegram as you noted.
They don’t even bother reporting Signal usage, it’s a round off error: https://www.statista.com/statistics/258749/most-popular-global-mobile-messenger-apps/
Telegram has almost 1 billion users, half as many as WhatsApp