Still changing. Staying the same.

A closed system can exchange energy with its surroundings, but not matter. In a reversible reaction, products can react to regenerate reactants. A sealed container allows both processes to continue.

At dynamic equilibrium the forward and reverse rates are equal and non-zero. Concentrations become constant, but they do not have to be equal. A flat concentration graph therefore does not mean the particles have stopped reacting.

The model below represents A ⇌ B with a fixed total particle count. Change the relative preference for B and follow the new balance. Particle motion is schematic; the curve uses a reversible first-order model.