Layering questions have a single answer that covers most cases: apply products from thinnest to thickest. A watery essence cannot penetrate a layer of occlusive cream, but a cream applies perfectly well over a serum.
The usual order
Cleanser, toner or essence, serum, eye product, moisturiser, and — in the morning — sunscreen last. Oils, if you use them, go after moisturiser rather than before, because they are the most occlusive thing in the routine.
Why serums go early
Serums carry the highest concentration of active ingredients in the lightest possible base. That is the entire point of the format: get the actives onto clean skin with as little in the way as possible. Burying one under a rich cream wastes the most expensive step in the routine.
Damp skin is not a myth
Humectants such as glycerin and hyaluronic acid draw water in. Applying them to slightly damp skin gives them water to work with. Applying them to bone-dry skin in a dry room can, occasionally, have them pull moisture from the skin itself.
The exceptions
Sunscreen always goes last, regardless of texture. Prescription retinoids are usually applied to dry skin, sometimes over a moisturiser deliberately, to slow absorption and reduce irritation. And if a product’s instructions contradict the general rule, follow the instructions — the formulator tested it.
Wait a minute or so between layers if you have time. If you do not, apply them back to back; it is better than skipping steps.