I'm pretty sure most iOS users were at least once aware they were using Apple's messaging service because they were required to initially sign into iCloud (or create their iCloud account for the first time) and explicitly opt into iMessages.

But because the service and application are reliable and consistent and because upgrading between iPhones is also mostly seamless and carries those settings forwards between device upgrades, it just fades into the background as something you did 5 years ago and don't need to think about again.