The fibrous tissue of the pectoral muscle group develops horizontally, and the inner and outer muscles you mentioned coexist at the same time. You can only practice the upper and lower parts. The most effective exercise methods are dumbbells and barbells, but an adjustable exercise bench with a slope slightly narrower than the shoulder width is needed.

Dumbbell chest exercises

Supine bird: Hold a dumbbell in each hand, lie flat on a flat stool, put your arms straight on your chest, palms facing each other, knees apart, feet on the ground, and stabilize your trunk. Slowly separate your arms downward. The dumbbell should be lowered below the body level, so that the pectoral muscles can be fully opened. Then, clamp the chest hard, so that your arms are close to the top of the chest. When the arms are separated to the side, the chest should stand up and the elbow joint should be moderately bent.

Birds inclined upward develop the upper muscles of the pectoral muscles (the head is higher than the knees), while birds inclined downward develop the lower muscles of the pectoral muscles.

Barbell bench press is also very effective, and the method refers to the dumbbell contact rule.