The rule you have heard is thinnest to thickest. It is a decent rule of thumb and a poor explanation, which is why people apply it anxiously and worry about getting it wrong.
The actual mechanism
The order matters for one reason: a layer that forms a film will slow down whatever you put on after it. Silicones, oils and heavy occlusives all form films. Watery serums do not.
So the point of thin to thick is not that thin products are somehow more important. It is that if you put your moisturiser on first, the serum after it has a harder job reaching skin.
Where it genuinely matters
- Sunscreen goes last. It is tested as a film on the surface. Anything on top of it, or mixed into it, disturbs that film.
- Anything water based goes before anything oil based. Water will not cross an oil layer well. Oil crosses a water layer fine.
- Hydrating layers go on damp skin. Humectants pull water from wherever they can find it. On dry skin in dry air, some of that comes from your own skin.
Where it matters less than people think
Two serums of similar texture, in either order, will not produce a different result. Neither will swapping which goes first between a niacinamide and a hyaluronic acid. If a routine falls apart based on which of two watery layers went first, the routine was not doing much anyway.
The one that is worth respecting
pH-dependent actives. Pure ascorbic acid needs an acidic environment, and layering something alkaline directly on top blunts it. In practice this means give vitamin C a minute to settle before the next layer, not that you need a timer and a spreadsheet.
A working order
Cleanse. Any water-based treatment. Any oil-based treatment. Moisturiser. Sunscreen in the morning.
That is it. Four to five steps covers almost everyone, and a routine you finish beats a routine you admire.