The amount of water you add to the dough affects how open the crumb is in the final result (open crumb means bigger holes and a softer texture). The higher the water level, the more open the crumb will be. The caveat to that is that a wetter dough is far more difficult to handle.
Soaking whole grain flour overnight before adding it to the final dough mixture will allow the bran in the wheat to soften and become more flexible. Then, when you add it to your recipe, it won’t affect the gluten as much by cutting the developed strands and losing all the gas build up.
Bread recipes need A LOT of strength to rise properly. And for sourdough, that strength is going to come from your starter. If you have made a starter from scratch, it should be at least 2 months old for it to be strong enough to rise bread properly.
Mixing baking soda into the dough at the shaping stage (just after the bulk ferment) will give sourdough bread an extra boost and help it become lighter and more airy. Baking soda is a heavy alkaline and reacts with the strong acidity of sourdough. The reaction gives off gases that help decrease the density of the dough. Sometimes you can see the reaction before your very eyes!