Im 20 year old male and I have a small jaw/chin/not defined, maybe related to overall lower t levels?

You won't see much change in your chin and jaw shape after 20 years old regardless of whether you supplement with testosterone. The gold standard options for jawline and chin definition are jaw implants and possibly liposuction for people with some extra fatty tissue that blunts the jaw and neckline. I recommend that you seek a consultation with a highly experienced plastic surgeon if the size/ shape of your jaw is a great concern for you.Stephen Weber MD, FACS

Do not expect too much growth in your jaw if you have not seen it already by your age. Generally, the jaw will grow in proportion with the rest of the face, with the greatest growth during puberty. If you have a tendency towards a small jaw this proportion usually does not change. Surgery is an option, either genioplasty where the jaw is cut and advanced, or a chin implant. Testosterone is not going to stimulate jaw growth. Consult a surgeon with considerable experience in facial surgery.