The new hair that sprouts up during puberty comes in thicker, darker, and often curlier than the rest of the hair on your body. Eventually, it will sprout under your armpits and in your groin, and it will also thicken and darken on your legs, maybe your arms, and likely your face too (at the mustache area, the brow line, and for guys, along the beard), but not all at once.
Hair changes are ruled by a group of hormones called the adrenal androgens - these are cousins of testosterone made in the adrenal glands that sit right on top of the kidneys. And here’s the deal: adrenal androgens do their own unique thing in every body, so sometimes hair will sprout before any other noticeable changes of puberty appear (like boobs, for instance), and sometimes it doesn’t grow until the end of the body transformation process. Hair can appear in the armpits before the groin or vice versa; it can thicken on the arms and legs before it pops up on the face, or in the reverse order. While growing new hair is a very predictable part of puberty, the when and where is not predictable at all.