SMS and MMS are completely integrated here in the US, you have no visibility into wether it's an MMS or SMS you're sending/receiving - it's just "messages" in my experience.

So if you take a smartphone photo, and send it to a phone number contact in a message, boom, you just sent an MMS.

A majority of random american public wouldn't recognize the MMS acronym, despite likely using it regularly.

sending "pics by SMS" is something people wouldn't even expect to work here in the UK in my experience (and it probably doesn't work for most of them, several networks/plans don't support them or bill them separately)

visiting the US back in the day I realised just how different the "phone culture" was, for instance there was such a thing like being charged for receiving calls (in ~2008) and drastically different coverage maps, etc - I have no idea how things are now over there

Basically it's the data plan that matters. The call time allotments are so large that they don't matter anymore.

Coverage is still an issue.

Group messages are also (usually) MMS.

if you say "group messages" here in the UK, everybody will assume you are talking about whatsapp, or blackberry messenger (before)

one of the reasons why I suspect MMS went completely unnoticed here in the UK is that Blackberry was extremely popular rather early, and then Android ate it

I don't think I've ever sent a single group message over MMS