"Chemistry answers that H2 and O2 can be joined to form water under certain conditions, but doesn't answer the whys".

Chemistry does answer the whys beyond product yields, why certain elements react is vastly covered throughout organic, inorganic & physical chemistry . It is the basis of these sub-fields, and how this is determined is looking at orbitals, energy states etc...

Yes, there's an overlap of knowledge

So, Pauli's exclusion principle is Physics or Chemistry? I'd say it's "both".

Physical Chemistry. (its a real thing)

Ask yourself who invented the "Hubbard" (Pariser-Parr-Pople) model originally the next time you say that, or where the "Berry" (Mead & Truhlar/Longuet-Higgins) phase really came from. This notion that reductionism is a one way street is stupid.

"This notion that reductionism is a one way street is stupid."

I never said reductionism is a one way street.

But in simulations you usually work bottom-up (in abstractions). Of course, sometimes you can't do that, because it's overly complex.

My field (electrical engineering) has a lot of physics as well and I don't mind if those who invented the techniques I use were physicists or engineers (or something else), or if such and such things are in the "engineering" domain or the "physics" domain, I just use them. The point is moot.