Lemon zest gives a dish a lemony smell without the same amount of acidity or water as juice. That makes it useful when a sauce is already the right thickness but tastes a little heavy.
Wash and dry the fruit, then grate only the colored outer layer with a suitable tool. Add a little, stir, and smell as well as taste. Zest can be strong, especially in a small quantity of food.
Juice is more useful when the dish needs a sharper finish. Use it gradually, particularly in plant-cream sauces whose behavior with acid can vary.
You can use both, but they do not need to arrive together. A bean pan might take zest during the final stirring and juice at the table, allowing each person to choose how sharp their own bowl becomes.