It primarily depends on the type of sweat glands that are located in the different regions of the body. Within the armpits and groin area, you have what are known as apocrine glands. These particular glands are known for secreting sweat which is thicker in consistency and also prone to smell foul once it interacts with bacteria in the surrounding area.
The interaction between sweat and bacteria is simple, when you sweat, bacteria that is found within different regions of the body, begin to consume (literally eat) the sweat. The waste product then excreted by the bacteria is what produces the odor you smell. Different types of bacteria exist throughout your body, hence the different types of smells.
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