Muscles contract when they receive signals from motor neurons in the brain and spinal cord. As individual muscles are used, they are able to receive signals from more motor neurons, increasing their strength.
This is motor unit recruitment, i.e. increasing the number of motor units (motor neurons and their attached muscle fibres) used to activate muscles. This is why when you begin working out, you will likely notice a big increase in strength initially, as your body becomes better at activating its muscles.
Simply using your muscles will increase their strength quickly, but not immediately support equivalent muscle growth.Muscle growth occurs in the downtime between workouts, when the body is at rest. To increase muscle size, the stress put on your muscle must be more than it has previously experienced. Thus, the easiest way to create muscle growth is by lifting progressively heavier weights.
for example due to hormonal variables which can affect protein synthesis. This is why it is easier for men to grow muscle than women. Being aware of where to start with your workout, which weights to lift with which exercises, how to fuel your body, and when to rest is vital in order to be able to maintain consistent growth and avoid burnout. If there is no food for your muscles to grow from, they will not grow.
Your PT can help you understand better how your body works as you eat and exercise, allowing you more freedom to adapt your workouts to your body and your goals.