Welcome to the forum!
To me there are three basic categories of how you might manage a bird before you cook it. Of course there are all kinds of different details and nuances with these, but three basic categories. In any of these you need to keep it cool so bacteria can't grow.
First is aging. A few hours after death rigor mortis sets in. The meat gets really stiff. Aging is when you allow rigor mortis to pass. It can last different lengths of time with each different bird but two or more days isn't unusual. The best way to me to test is to wiggle a leg. If it is really loose then it is ready, If there is any stiffness wait some more. If you cook the chicken immediately aging isn't necessary but once rigor starts to set up, it is.
Brining is when you soak it in a salty solution. Brining is not about giving it a salty flavor, you can salt it in any part of the process if you want a salt flavor. Brining causes the meat to retain moisture. People can complain about cooked meat being dry. This is why the meat you buy at the store is generally brined, it helps reduce the meat drying out. How important this is depends a lot om how you cook it. If you use a dry method like frying or grilling this can help. If you use a wet method, say crock pot, pressure cooker, or in a stew it's not very important. As you would expect the strength of the salt solution and how long you leave it in the saltier the meat will be and the more moisture (up to a point) the meat will retain. There is some personal preference in that.
The third category is marinades. This is when you soak the meat in an acid (usually wine or vinegar based). The acid breaks down some of the tissue, which has the effect of tenderizing the meat. Of course the stronger the acid and the longer it is in there the more it tenderizes. If you leave a very young bird in marinade very long it might get mushy. But if you are cooking an older bird a good marinade can go a long way to make it palatable. Sort of the extreme on this is Coq au Vin which requires marinading in wine. This can make a gourmet meal out of a very old rooster. People add all kind of flavors with different marinades, that's what a lot of people think of when you mention a marinade. Some marinades add great and interesting flavors but the original purpose was to tenderize an older bird. That flavor is a bonus.
To me the easy answer on aging is when rigor has passed. That's pretty much a requirement for tender chicken. But for brining and marinading it's going to depend on the age when they were butchered, how you will cook it, and to a large extent your preferences. We all have different tastes and tolerances. There may be some trial and error to find out what suits you best.
Good luck!
