How does breast tissue migrate to the armpit area? (photos)

Hello.  This is a common location for extra breast tissue.  This area probably contains both fibrous glandular breast as well as fat.  Liposuction can remove the fat but not the solid breast.  Your right armpit, more than the left,  seems to have a bit of extra skin, so I would recommend an excision of both sides to have the best chance of the areas being flat after surgery without loose skin.  Blue Cross insurance considers this cosmetic in my state.  The risk of cancer is low, but it is a good idea to have your surgeon send the removed gland to the pathologist.

The growth under your arms may indeed be breast
tissue that has grown over time. It should be removed surgically and sent to a
pathologist to make sure that the tissue is benign. Consult a board certified
plastic surgeon to discuss your options for removing this tissue.

This accessory breast tissue may be removed with excision. It really depends on the surgical technique applied and the surgeon to determine the cost, but I estimate it would be a few thousand dollars.