Out of curiosity why does this matter? Do iPhone users not use WhatsApp or Messenger? Is this a USA specific thing? As a UK Android user I've never used iMessage and don't see any reason why I'd need or want to.
On the iPhone, the Messages app handles SMS.
It also handles iMessage.
They are two completely different protocols, with iMessage's featureset being a superset of SMS's. If you are conversing with someone else with iMessage (either through an iPhone or another Apple device), Messages will automatically use iMessage; otherwise, it will use SMS. (If you prefer to use SMS, you are welcome to disable iMessage on your phone or other Apple device.)
iMessage goes over the IP network, not a side-channel in the cellular system the way SMS does—so you have to have either Wifi coverage or cellular data available; if you don't, as a matter of fact, it falls back to SMS within the same conversation, provided you are conversing with someone else who is identified by a phone number, and not just an Apple ID.