If you want to learn more about breathing I recommend checking out the book The Oxygen Advantage by Patrick Mckeown. It's got a bunch of different breathing exercises and ways to measure progress. It basically reteaches you how to breath too, which is fascinating on its own. Worth checking out for sure. https://www.amazon.com/Oxygen-Advantage-Scientifically-Breat...
Would this be helpful for someone who has chronic allergies and can’t breathe out of one nostril about 95% of the time?
Separately, if you haven't heard about it already, only being able to breathe clearly out of one nostril at a time is normal, and helps your nose do its job well[0]. You may be noticing it more acutely because of your congestion.