system
Given the following post, respond as if you're commenting on Reddit:
human
What happens at a chemical level when a bottle of liquor is allowed to "rest"?

I'm curious about this and don't really see it addressed elsewhere.  It's become common to allow a bottle of liquor (whisky, cognac, etc) to "rest" after opening.  In fact, I just read this in a vodka review: "It is Beluga’s philosophy to have this vodka rest after each step of the production," explains Borisov. After a five-time filtration process to achieve maximum smoothness, the vodka's final resting period is 90 days. "This allows it to reach perfect balance and harmony,"

Since I always like to ask myself "why is that" and "what mechanism is at work here", I'm asking precisely that.  Is there some chemical change happening to drive a flavor shift or is this "resting" of a seemingly stable substance mostly psychological in nature.  If a sealed or capped bottled is allowed to "rest" sans additional oxygen or heat, how is it changing?
gpt
With a lot of spirits, their individual character comes from the infusion of other flavors based on what vessel they're resting in. Bourbon, for example, uses oak barrels that are charred on the inside. As they rest in large barns that are not temperature controlled, the natural thermal expansion and contraction causes the spirit to move in and out of the barrel wood, absorbing various organics and other chemicals along the way. 

Once bottled, spirits don't age anymore. Ideally. If they're poorly sealed, then they may oxidize. 

Resting a bottle after opening will allow for any volatiles that want to escape to do so, although in a narrow bottle your available surface area is limited. That's usually why you decant into a different container from the bottle before drinking. 

Any flavors remaining in the spirit after it rests are what 'should' be there. 

Vodka is a bit silly though, as it's generally going to be as close to straight ethanol and water as you can get.