> two completely different ways depending on whether it's an iMessage or a regular text

Isn't that true for any device? not just watches?

A "regular text" would be an SMS and those use Signaling System Number 7 telephony stack.

iMessage uses application level protocols sitting on top of IP in the standard OSI model?.

SS No 7 only roughly maps into OSI model, the equivalent application layer would use MAP - Mobile application part.