There's 3 main mechanisms for muscle growth that need to work together in order for you to not only grow the muscle but to also retain it!
1. Muscle Tension- In order to produce muscle growth, you have to apply a load of stress greater than what your body or muscles are used to. So, how do you do this? The main way is to lift progressively heavier weights either each set of your workout or as a larger goal from week to week or month to month. This additional tension on the muscle helps to cause changes in the chemistry of the muscle, allowing for growth.
2. Muscle Damage- This local muscle damage causes a release of inflammatory molecules and immune system cells that activate satellite cells to jump into action. Essentially saying that the muscles rip apart to grow back together. If you have ever felt sore after a workout, you have experienced the localized muscle damage from working out.
3. Metabolic Stress- Metabolic stress causes cell swelling around the muscle, which contributes to muscle growth without necessarily increasing the size of the muscle cells. This is from the addition of muscle glycogen. which helps to swell the muscle along with connective tissue growth. If you have ever felt the burn of an exercise or had the "pump" in the gym, then you have felt the effects of metabolic stress.
Building muscle takes time and consistency. Be patient with yourself and have a plan when working out so these movements your doing and the time your spending working on building muscle pays off how you intend it to. Muscle growth is best achieved through strength training. You want to create than mind to muscle connection- feeling the contraction of the movement with the weight to the muscle and challenge yourself as long as your form is good! Remember to eat quality amounts of protein for your body weight and stick with it. Your only limit is you!