How exactly does exercise relieve stress and tension? any particular form (cardio, yoga, etc.) best or is it a matter of personal preference?

Endorphin release: Exercise is good for the mind and the body. It helps condition the heart, the muscles, improves blood flow, improves oxygenation, etc. For the mind, it helps due to the natural release of endorphins that make you feel good, increase energy. What type depends on what you like to do and the consistency of doing it. Combination of things usually work for most b/c it prevents boredom.