Brown adipose tissue is present in small mammals (e.g., mice) and in newborn humans. Most of it disappears in adult humans. The cells in both types of fat are called adipocytes although they differ in origin, structure, and function in the two types of tissue.
On the left hand, index finger. It's not the index finger you wear it on. Left hand, ring finger. The finger that you would wear a wedding ring on.
After the wedding day, wear your engagement and wedding rings however you like them best. Again, the most popular way in the US is to have them both on the ring finger of the left hand with your wedding band first, but you can separate them and wear them on opposite hands, or you can wear your wedding band solo.