iMessaging isn't really a text/sms messaging app, in my opinion. The way I look at it, it's a propriety messaging app for between Apple only devices, one that uses a propriety messaging data protocol for such purpose and for sending/receiving pics, reactions, emojis, gifs, videos, etc... For all that, one may as well compare it to WhatsApp or FBMessenger or Signal or Telegram (or even RCS), as any other data using app (ie: not an sms/mms/texting app). Then, iMessaging also just happens to also have this other thing hanging off the side of it, which also allows for sms/mms/text messaging between phone numbers for if an Apple device was to try to communicate with a non Apple device.