In iMessage, which seems to be a thing in the USA (I don't know of any other market where it's a dominant messaging system), non-iPhone users participating via SMS appear to iPhone users with a different (green) background color to their message bubbles.
Your own messages appear in a different color, not theirs. At least not on current iOS.
It's so you know that the messages you're sending are going over MMS instead of iMessage, so zero features other than sending plain text to individual recipients will work well, if at all. It's like time traveling back to the late '00s with text-chat-groups inexplicably splintering mid-conversation, adding new participants screwing everything up, and potato-quality photo and video attachments that take forever to download and simply fail a good fraction of the time.
My friend group solved this by switching to WhatsApp. But if we'd all been iPhone users I doubt we'd have bothered, because iMessage is at least as good and, if you use some of the more-advanced features, much better, provided your chats are actually going over iMessage and not MMS.
iMessage is basically exactly the same as having a good messaging app just for iPhones plus a necessarily-shitty MMS one, except you don't have to switch between them because they're combined.
I distinctly recall Google trying to do the exact same thing when I was an Android user, must have been over a decade ago: on a major OS update, they replaced the texting app with some unified MMS/other-messaging-service (who could possibly keep track of them all? May have just been gchat, at the time) app. I had to "downgrade" to the old texting app because the new one was so broken that it was unusable. In particular, it lost messages all the time, or would switch mid-conversation between MMS and other-messaging-protocol, screwing everything up. iMessage is nowhere near that bad, at least, even when in "green bubble" mode.
