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Your chest, or pecs, is comprised of two different muscles. The pec major, the large part of the muscle on top, and the pec minor, the smaller muscle underneath.
Additionally, the pec major is comprised of the clavicular head (top of the pec) and sternocostal head (bottom of the pec) that connects everything together and to other parts of your chest.
You can't isolate your "lower chest." Just like you can't isolate your upper chest or inner chest because any time you do a pressing or fly motion, you are going to be activating your entire pec major along with other assisting chest muscles. You can, however, place more emphasis on your lower chest with certain exercises.
Decline bench presses, decline flies, and chest dips are a few exercises that will put more emphasis on the lower portion of your pectorals. However, regardless of exercise, your entire chest and pectorals will be worked.
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