Other people have given definitions of discrete and integrated circuits, so I am not going to repeat it here. But, you CANNOT physically put multiple discrete circuits on one plate of Silicon and call it an integrated circuit. It is not how integrated circuits are made. Integrated circuits are made up of silicon wafers, not placed onto silicon wafers. So, you will have to design your circuit keeping in mind all the discrete circuits and then process it on silicon (hundreds of processing steps) and you can create an integrated circuit. But then again, some discrete circuits may not be possible to be created on silicon wafers as a part of integrated circuits.
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