Ring flies for the chest and scapula pulls for the back. You can also do these movements with resistance bands or cables!
The movements demonstrated here will solely target the chest and back muscles, because you are taking your arms out of the equation.
If your arms are very strong, it's possible you have been doing many exercises like chin ups, rows and push ups dominantly with your arm muscles - while the chest and back have been partly neglected.
These exercises are done with gymnastic rings, which is one of the best tools available, but you can also do similar exercises with resistance bands or with the cable machine.
With the gymnastic rings, just adjust the height of the rings to your level of fitness. This way you can do all of these exercises even if you are a beginner.
Ring Fly Exercises
Upper Ring Fly
Overhead Ring Fly
Inverted Scapula Pull
Overhead Scapula Pull
Combine all of these into one workout and you are all set! You will work both the anterior and posterior side of the body and even your abs (with the overhead scapula flies).
Take 1-2 minutes of rest between sets and exercises.
Train hard, stay safe.