Which products you should use together and shouldn't use together.
Ah, I hear you! I think this one is made more confusing than it needs to. The main thing to watch out is irritation: too many actives used in the same step or on the same day or even week can lead to skin irritation. But it is not caused by interaction of the ingredients - it’s just getting too much for our skin in general. We should be especially careful with using other actives with acid exfoliants (exfoliation weakens the skin barrier and increases the risk of irritation for any additional active applied on top).
In terms of effectiveness and ingredient interaction, the general rule is to let the product sink into the skin before applying the next one (with the exception of a moisturizer &oil - you could apply an oil before the moisturizer is fully absorbed). For absolute majority of the products and ingredients this rule takes away the risk of any interaction/deactivation.
The only “capricious” diva skincare ingredient in this regard is benzoyl peroxide: it shouldn’t be combined with ascorbic acid and other antioxidants, and also not with retinol in one step (e.g. am or pm).