You could actually bring your hands together without using any chest at all
The lower arm is largely irrelevant
Your pectoralis major is the large, fan-shaped muscle on either side of your chest
It has the primary function of bringing the upper arm towards the midline of your body
Due to the upper arm being the key attachment point, your focus should be on driving the upper arm towards the midline of the body
You can see this for yourself by doing this simple exercise right now
Stand or lay down with your arms outstretched & elbows fully extended
First, simply bring your hands towards one another by bending (flexing) at the elbow – you’ll note that you feel nothing at all in your chest
Now, go back to that same starting point, keep your arms outstretched & bring the elbows towards one another – you’ll now feel this solely in your chest
Apply this feeling whenever you perform any chest movement to maximise the contraction & stimulus