Bro I'm a PT so Ill help you out there. Even though Im worth money Ill do ti for free sinse you are one of the bros that works out.
Well first of all there are 2 sections to your "pec" ("pec" is your chest) and the incline works your "upper pec" more than your "lowere pec" and flat works your "lower pec" more than your "upper pec".
So basically its all really complicated and hard and you should get certified like me, but bascally your "upper pec" is weaker then your "lower pec". But also incline also uses more shoulder plus the "upper pec". If you work your shoulders more it will help your "upper pec" to.