I'm going to try my best to explain 90° rotation:
So, you know that if you rotate something 180°, it's completely flipped (think about spinning around half-way).
Or if you spin something 360°, you spin around the whole way and end up in the same spot that you did when you started.
Notice how 90 is actually 1/4 of 360.
So imagine spinning instead of 180, spinning half of that. so you barely rotate. That's exactly what you're doing to this shape here. and if you do it about the origin counterclockwise, the origin is (0,0) so I drew it in Quadrant III, as you can see in my attachment.
You can see that every point has been moved by 90°, I put all of the variables there so you could visualize it better!
I hope this helped, let me know if you have any questions! :)
