iMessage is kinda both its own messenger and SMS client. So when you text a number, I don't think you have a choice of how to send it. If the user is "Apple" it sends as an imessage, if not message, it turns the bubble green (so you know its SMS). Though in the epic trial Apple sees messaging as a "lock in feature"
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In another exchange, Apple executives discussed in 2013 whether releasing a version of iMessage for Android would make it easier to switch phone brands. iMessage is still exclusive to Apple products.
“I am concerned that the iMessage on Android would simply serve to remove and obstacle to iPhone families giving their kids Android phones,” Craig Federighi, Apple software senior vice president, wrote in 2013."
" In another exchange, Apple executives discussed in 2013 whether releasing a version of iMessage for Android would make it easier to switch phone brands. iMessage is still exclusive to Apple products.
“I am concerned that the iMessage on Android would simply serve to remove and obstacle to iPhone families giving their kids Android phones,” Craig Federighi, Apple software senior vice president, wrote in 2013."
https://www.cnbc.com/2021/05/03/epic-games-v-apple-trial-app...