The inner chest muscle is the sternocostal head of the pectoralis major. The best exercise to target it is the wide grip push-up.
Chest Exercises at Home, Upper Mid Lower Chest
What Exercise Works the Outer Pecs?
What Are the Benefits of the Tricep Extension Exercise?
Superset For Inner Chest Muscle Building (ft. Scott Herman)
How to Do Incline Pushups: Techniques, Benefits, Variations
How To Do The Push-Up Properly
Decline Push Up, What Muscle Does It Work?
Do you workout the core of your chest by doing push ups with your
Target INNER Pecs - 3 Flat Dumbbell Chest Exercises
The Perfect PUSH-UP Workout (3 LEVELS)
The Muscles Used in a Pec Deck Machine