Shouldn’t the electric field inside a slab of charge be zero?

In one of the problems involving Gauss’s law, we have to calculate the electric field outside and inside a symmetric slab of some thickness and certain net charge. Of course, the electric field we get outside the slab is independent of the distance between the point and the slab which is understandable but the solution continues by calculating the electric field inside the slab, how is this possible?

In electrostatics we are dealing with non moving charges and if we have a bunch of charges next to each other the only possible way for them to still stationary is that the force affecting each one of them is zero (or to say the total sum of forces is zero) but if we have an electric field inside the slab that means the charges are being acted on by a force and hence disrupting the equilibrium of electrostatics.

So how can we have an electric field inside a slab and still have static charges that are standing still and in equilibrium?

To give you a good idea about the kind of problem i’m talking about look at this problem: https://youtu.be/1eAauKHR72A

Notice that we have already assumed the electric field is only in the upward and downward direction (meaning that the left and right direction have been canceled by each other to preserve the equilibrium of motion)