I am “tech literate” (I’m on this forum, after all) and it took me some good years until I learned that iMessage is not SMS, i.e. when WhatsApp stopped working

I am “tech literate” (I’m on this forum, after all) and it took me some good years until I learned that iMessage is not SMS, i.e. when WhatsApp stopped working on my non-updated iOS version and I had to switch to sending photos via “SMS” (which is in fact not SMS). Very recently I’ve also discovered that if I ignore the “log in to iTunes/iCloud” messages for long enough then I won’t be able to send “SMS”-eses anymore, which, when you think of it, technically makes sense but which I found puzzling nonetheless and it took a few hours of head-scratching to figure out (it’s not very intuitive to think that you need to log in to a different platform in order to send messages through the same interface that I had used for more than 20 years before without ever having to do that).