Equilibrium is dynamic, not still. Both reactions keep happening at the molecule level. Because they happen at equal speed, there is no overall change in how much of each substance is present.
Chemists measure how far a reaction leans toward products or reactants using the equilibrium constant, K. A large K means mostly products are present at equilibrium. A small K means mostly reactants remain. Le Chatelier’s principle predicts what happens when something disturbs a system at equilibrium.
If you add more of a reactant, raise the pressure, or change the temperature, the reaction shifts to partly cancel out that change. This idea helps chemists control industrial reactions to get more product. Equilibrium can be reached from either direction.
The system can start with pure reactants, pure products, or any mixture in between. It still settles at the same ratio of concentrations, as long as the temperature stays the same. A common misconception is that equal concentrations of reactants and products signal equilibrium.
That is not true. Equilibrium only requires equal rates. The actual concentrations at that point depend on the size of K, which can make products or reactants far more abundant than the other.