Those yellow underarm stains that appear on your white shirts are caused by a reaction between the aluminum in your deodorant and your sweat. They can be really difficult to remove.
Here’s a simple pantry combination to try for white washable fabrics.
Soak the area in a mixture of equal parts water, baking soda, and peroxide (about 1/4 cup of each). Gently scrub with a toothbrush to remove any particles then wash as usual.