If an object is in equilibrium it is a clue to the nature of the forces acting on it. A single applied force cannot cause equilibrium unless it has a zero value, but then it isn't there in the first place.

It takes at least two forces to create a condition of equilibrium. If there are only two forces then they must have the same magnitude and be in opposite directions. This is the only way in which their sum could be zero.

Action and reaction forces cannot cause equilibrium. Since they are operating on different objects, they cannot be added together. Some third force may be present to oppose an action or reaction force.

If you kick your locker, your locker exerts an equal force back on your foot. Your foot changes its motion; it stops. The locker doesn't move because your kick did not overcome the other forces holding the locker in place. If it does, the front of the locker would start to move (a dent).

Knowing an object is in equilibrium allows you to find unknown forces. For example, if it is known that the forward force on your car is 2000 N, but the car is moving down the road at a constant velocity (equilibrium) then the frictional force on the car must be 2000 N in the opposite direction.

Suppose two elephants are pulling on a shirt, one exerts a force of 35 N [0°] and the other a force
of 55 N [90°]. What force would you have to apply to the shirt to keep it atationary?

Keeping it stationary means the shirt would be in equilibrium. The total of the three forces (the two elephants and you) would have to be zero.

First, find the sum of the forces from the elephants.

Your force would have to be equal in magnitude to the elephant's combined force and opposite in direction.

The object of many aspects of engineering is to create equilibrium. Knowing the basis of the cause of equilibrium and how to calculate numbers in its application goes a long way to working in this field.