Remember that the planes intersect at infinitely many points which make a line. In the second picture they just choose y=1 because it's an easy number and they find the values of x and z which intersect both planes and y=1
Remember that the planes intersect at infinitely many points which make a line. In the second picture they just choose y=1 because it's an easy number and they find the values of x and z which intersect both planes and y=1