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.
Keeping your arms fixed (slightly bent) will allow you to focus only on the chest and back muscles which will then translate into more strength and bigger size in these areas.
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
Ring Fly

Ring fly will isolate the chest. It's possible to keep the arms straight but this will put lots of stress on the tendons.
As long as you feel the exercise primarily on the chest muscles, it's perfectly fine to keep the arms bent.
Upper Ring Fly

Upper ring fly eliminates all involvement of the anterior deltoid and focuses more on the chest. Trapezius muscles are also activated.
Overhead Ring Fly

Overhead ring fly will work the chest, lats, triceps and traps. You will feel the exercise the most in the abdominals though, because you are basically doing a ring abs roll out.
Scapula Pulls
Inverted Scapula Pull

Focus on pulling your shoulders behind and your chest up. Inverted scapula pulls will heavily target the middle back, but also the posterior deltoids.
Overhead Scapula Pull

Overhead scapula pulls will focus on the upper traps, but the middle back and posterior deltoids are also worked a lot.
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.