I understand that the wing of the plane creates downforce, lifting the plane into the air, so how do planes fly upside-down? Wouldn’t the wings start pushing the plane down into the ground once flipped?
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Any surface with a reasonably sharp trailing edge can generate lift with a positive angle of attack. The sharp trailing edge is important because it causes the flow to separate cleanly (the Kutta Condition). There will be a range of angles of attack from somewhat positive to somewhat negative where it will work well, and the lift increases/decreases (including being negative) with angle of attack. If that range is exceeded, the flow will separate from the suction surface (the one that has low pressure) leading to stall.
for a straight section with no camber, at zero incidence it will produce zero lift, and positive lift at positive incidence, and negative lift at negative incidence. This kind of neutral symmetrical section is useful for things like the vertical stabilizer.
If the airfoil section has camber (curvature so the wing bows upwards from leading to trailing edge), it will be biased towards producing positive lift at zero incidence, but if the incidence is sufficiently legation, it will still have a regime of producing negative lift at sufficiently negative angles of attack (meaning positive lift for an inverted aircraft). It will be less efficient (more drag) and have less range (ie the lift can increase with angle of attack less before stall), but it is sufficient to allow a plane to fly inverted.
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