A square is a rectangle. A rectangle is a parallelogram. And a parallelogram is quadrilateral. A quadrilateral is a polygon with four sides. The definition of a polygon is “A plane figure bounded by three or more straight lines”.

(1) These lines are not straight, and

(2) Two of the angles are actually 270°, and only their exterior angles are 90°.

(3) You cannot say the right angles are 90° because one leg is a curved line. To say a curved line is a right angle is not possible in two dimensions.