I believe all the international signs have unique shapes and the stop and yield signs are borrowed from there.

No, only the yield and stop signs, and the right of way sign (diamond shaped) have unique shapes. Oh wait, the one way sign too, being a long rectangle. Other signs are shaped according to group: Warning signs are triangular, tip up, signs forbidding or mandating something are round (distinguished by color), and informational signs are rectangular. And one interesting case is the do not enter sign, which is usually curved. That makes it more obvious to someone looking for a turn, but also you can recognize it from behind so you know to go in the far left lane if you plan to make a left turn.