AoA is measured relative to the incoming airflow. In a straight wing in straight and level flight that is relatively easy as the entire wing is seeing the relatively (to the plane) stationary air coming at a single velocity and direction**.
In a propeller the airspeed changes along the length because the velocity of the propeller changes along the length.
To make the numbers simpler - suppose the tip of the propeller is going 100 mph due to rotation; the center of the propeller will be rotating at 0 mph.
Suppose further that the plane is flying at 100 mph.
Note - for most propellers these numbers don't work; however the method is the same for whatever airplane/propeller combination is being designed. Also I am ignoring that some propellers operate with tip speeds near the speed of sound where Mach/ compressability effects become significant and shock-wave formation alters the performance. Which is why it's better to buy a prop from a maker that has worked out all the particulars.
At the tip the propeller is going across wind at 100 mph and into the wind at 100 mph, so the relative wind is at 45º to the tip and going 140 mph. This is the 0º AoA basis for the tip.
At the center the propeller the prop is going 0 mph across the wind and into the wind at 100 mph, so the relative wind is at 90º to the tip and 100 mph. This is 0º AoA basis for the center (though few props get all the way to the center.)
The pitch of the prop will change/twist from 90º to 45º along the length of the blade from center to the tip just to maintain a 0º AoA at this condition. Change the airspeed or the tip speed and get a different angle at the tip. The center is always 90º.
**in low speed, <0.5M, in straight and level flight there is an upwash coming from ahead of the plane so the geometric AoA seen from the path of the plane is not the AoA that the wing encounters. It's not a great deal, but it is why ground-effect happens.