> Is this some sort of chemical reaction?

No, it's dramatically more complex than a chemical reaction, see https://en.wikipedia.org/wiki/White_hole#Big_Bang/Supermassive_White_Hole - "The Einstein–Cartan–Sciama–Kibble theory of gravity extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, the torsion tensor, as a dynamical variable.
Torsion naturally accounts for the quantum-mechanical, intrinsic angular momentum (spin) of matter.
According to general relativity, the gravitational collapse of a sufficiently compact mass forms a singular black hole.
In the Einstein–Cartan theory, however, the minimal coupling between torsion and Dirac spinors generates a repulsive spin–spin interaction that is significant in fermionic matter at extremely high densities.
Such an interaction prevents the formation of a gravitational singularity.
Instead, the collapsing matter on the other side of the event horizon reaches an enormous but finite density and rebounds, forming a regular Einstein–Rosen bridge.
The other side of the bridge becomes a new, growing baby universe.
For observers in the baby universe, the parent universe appears as the only white hole.
Accordingly, the observable universe is the Einstein–Rosen interior of a black hole existing as one of possibly many inside a larger universe.
The Big Bang was a nonsingular Big Bounce at which the observable universe had a finite, minimum scale factor."

And that's just a postulate/hypothesis, not something we have any definitive evidence for, or can even test with current technology.

> Secondly do we not know as a species why black holes/ white holes are able to exist.

We know many (but not all) details of how black holes form and how they behave - we predicted them, then found a bunch with our telescopes.

I believe there's some contention about how the Pauli exclusion principle gets squashed when a neutron star gets big enough to transition, but perhaps I'm simply not well-read enough.

We do not know what happens beyond the event horizon, although there are several competing ideas even amongst the highest echelons of cosmological theorists and pure mathematicians.

> Third question, have we ever observed a white hole colliding with a black hole.

We're not convinced that the big bang is a white hole, but if it is, it's the only one we've ever seen - there's no evidence that there are (other) white holes in the universe even though they show up in our math.