While I understand the principle behind a P-trap, I fail to understand why gasses downstream cannot pass through the liquid held in the trap? Gasses can bubble through and/or become soluable. Is the basis that these drawbacks to the P-trap are substantially minimized?
Because there is no pressure to push the gas thru the water seal.
And (one) reason why there’s no pressure is that a waste system that’s up to code is vented so the gases have a place to go.
Exactly. The traps (also known as a seal) do not work if the associated vent becomes blocked by build up of leaves, bird’s nest, hair, etc. Also, in very high winds, it is possible to get enough pressure/vacuum to either push gas past the seal, or to suck the water out of it so it no longer functions.
Gasses can bubble up if released underwater. It can’t dive into the water from the surface on its own and come up the other side.