I’ve heard that you don’t actually breathe out of both nostrils at one time, but that actually you switch between them. It seems like it’s true… so my question is first is it and second what’s actually happening that causes the switch if so? Does it gradually changes or is it “breathe out left nostril, breath in left, breath out right”
As long as both nostrils are clear, approximately equal airflow goes through each one. There is no mechanism in place to switch between them, although sometimes one side will become congested while the other remains clear. You could verify this for yourself but holding a hair or thread under each nostril simultaneously and watch in the mirror as you exhale through your nose. If you’re a smoker, you can exhale smoke instead.
Actually, from what I can tell (holding my hand under my nose and feeling the air) I do tend to breathe out noticeably more from one at a time. I wouldn’t have thought it would be a blocking problem unless my nose is spontaneously getting blocked in one side and then another every so often.
Left at the moment, btw. 
Mine seems about even. If I close one and blow through the other, it feels like there is slightly more resistance in the right one than the left. I do have a tendency, when I get a cold, to become clogged on one side while able to breathe through the other. If I lay on my side with the clogged side up, drainage will eventually cause the sides to swap.
The OP is thinking of the nasal cycle, in which the mucous lining of the nostrils become engorged in an alternating rhythm. Google will turn up much more on the phenomenon if you are interested.
Often what we describe as ‘clogged’ isn’t actually that at all - it’s constricted by swelling due to infection or irritation (although this does of course mean it’s easier to become clogged with mucus)