there are water-based serums and oil-based serums. Water-based serums have molecules that are tiny enough to penetrate to the deeper levels of the skin. Whereas, oil-based serums go on top of the moisturizer because of their bigger molecular size. They’re supposed to keep the creams and whatever other products you apply underneath moist. When you use an oil-based product containing a high amount of lipids, you increase the skin’s ability to prevent water loss.
Here’s a typical sequence:
3. Medication (if you use any) spot treatment
4. Anti wrinkle cream- serum (thin)
5. Moisturizer or Night Cream.
Remember that it’s critical to listen to your skin, get good professional advice, to find out what works best for you.
I hope this helps!