Just about everybody I know uses both. The pattern I see is that groups associated with some entity like a company or a club are on signal, because at some point (before Musk took over as chief billionaire villain) it became a thing to not have your brand associated with Meta, while many groups on the more the friends&family end of the spectrum remained on Whatsapp. And 1:1 conversations are simply on whatever messenger they started on. Which is super convenient, because of the way the Android multitasking model works (and I suspect it's similar on the other side?). Far more convenient to switch between a Signal, a Whatsapp and the occasional Facebook Messenger than between two WhatsApp groups, worse if those are Whatsapp groups with a channel hierarchy.

(Telegram: even just having it installed would be seen as a political statement, it's where the weirdos congregate to antivaxx and flatearth and reinstate monarchy or whatever they do)