iPhones implement SMS/MMS, which are both standards set forth by the GSM specification. That's the level of message exchange the underlying protocol offers. RCS is the "next gen" SMS, which is also being implemented. SMS/MMS/RCS does not support E2E encryption.
Apple then offers iMessage as a seperate service, on par with WhatsApp, Signal, Facebook Messenger, etc. It is literally just a messaging service running over the internet, using some form of identifier to identify you, which might be your phone number, like signal, or your email.
iMessage also offers proper E2E encryption, which is hit or miss with competitors. The challenge with E2E encryption is peer discovery. iMessage has made encryption easy, to the point that nobody thinks about it, but that's really what the blue bubble indicates, that your message is E2E encrypted.
There is literally no monopoly there other than Apple offering the superior tool. There is open communication with other phones, using SMS/MMS, which is the lowest common denominator when you're talking phones. That is literally the level of capability you can guarantee when talking over GSM.