But, I've been
asking myself for a long time that if this is so, why airplanes can fly
inverted or at a 90 degree bank angle.
Symmetrical aerofoil produces as much force downwards as it does upwards. A cambered wing (which produces more up force than down at 0 AoA) can still fly inverted, but must choose a high AoA to balance the forces.
Understand, the cambered planes you fly... the bottom surface also produces "lift" towards the ground, its just that it's overcome by a greater uplift of the upper surface.
E.g Acrobatic/Military planes use a more symmetrical wing to help with this as one advantage.
Planes can fly at extreme attitudes only when they have powerful engines where pure thrust is used to overcome the lack of vertical lift from the wing (because its at 90 degress).
A red bull racer can fly straight and level at 90 degrees bank. The symmetrical wing of such an acrobatic plane means it hardly changes heading when banked (achieved with elevator input), whilst the huge thrust of the engine compensates for the weight.