> I realize my perception is skewed by living outside the US

I too live outside the US, and never use SMS for communicating with another human. It's all WhatsApp/Signal etc. WhatsApp is ubiquitous.

I'm curious — do Americans use SMS a lot (as opposed to things like WhatsApp)?

I was in US a few years ago, and from what I gathered most people did not use WhatsApp. Don't know if the situation has changed.