Every other product on our formulas page is described somewhere in terms of the layer it is meant to reach past the surface. Sunscreen is the exception, and it is worth explaining why that is a feature rather than an oversight.
Zinc oxide and titanium dioxide are physical (sometimes called “mineral” or “inorganic”) UV filters. They work by sitting on and just above the corneum and scattering or absorbing UV radiation before it reaches living cells beneath. That mechanism only works if the particles stay put — a mineral filter formulated to “absorb deeply” would be a contradiction, since its entire protective function depends on intercepting UV before it gets past the outermost layer.
This is different from how a chemical (organic) UV filter works — those absorb UV radiation through a chemical reaction within a thin film on the skin surface as well, though by a different mechanism at the molecular level. We formulate with physical filters only in our current SPF, a formulation choice, not a claim that chemical filters are unsafe; both categories are permitted and regulated in the markets we sell into.
The practical implication for the rest of your routine: SPF goes on last, over everything else, specifically because it is not supposed to absorb into what came before it. A serum applied on top of sunscreen is applied on top of a surface film that is doing its job by staying there — which is also why we describe our own mineral formula’s finish honestly (it can leave a visible cast on deeper skin tones) rather than promising an invisible finish we have not actually reformulated to deliver.