The difference between serum, essence, and ampoule

When we usually do skincare, we typically use the order of toner, lotion, essence, and cream, but sometimes a serum or ampoule is used instead of essence at the essence step.

So, what is the difference between essence, serum, and ampoule?

It usually lies in the difference in concentration and nutrition.

The essence has the lowest concentration, followed by the serum, and then the ampoule.

Since ampoules contain the most concentrated nutrients, they are expensive and are often used as care for special occasions.