Well, it depends. Let me give you an example.
Take orange and avocado. Both fruits provide nutrition but in different forms. Oranges contain vitamin C while avocado benefits the body through monounsaturated fats. Both are good for the body, but the functions and benefits are different.
Now imagine your skin like a chocolate cake with three layers. The first layer is called the epidermis; the middle part is called dermis, and the base is called the hypodermis. These three layers have different biological structures.
The top layer (epidermis) is an oil-based and acts as a waterproof barrier. It prevents bacteria and allergens from entering the second layer and locks water in the skin.
Once you start going to the bottom layers, the biological structure changes. The skin cells and their surroundings are mostly water-based. Compared to the epidermal layer which consists of dead cells, the cells in this lower tier are alive.
Now let me tell you the main difference between serum and the moisturiser. Mostly the serum is water-based while moisturiser is water-based AND oil. This means serum can absorb into the skin to go to water-based skin cells while the moisturiser will stay in the skin's surface area.