You're confusing two different things, though I don't blame you for it, as it is confusing. "iMessage" is the OTT E2E-encrypted chat protocol. "Messages" note the lack of the leading "i" and trailing "s") is an iOS app that lets you send and receive messages using both the iMessage and SMS/MMS/RCS protocols.

iMessage is not open, and Apple fights efforts by other companies (e.g. Beeper) to interoperate with it.