Testosterone levels are one of the most misunderstood medical subjects, mainly because they're really at the heart of what I'd call "internet science". You cannot just perform a medical evaluation of someone by directly saying "they found X is correlated to Y, therefore more X more Y." Human bodies don't work like that at all, least of all hormones.
Hormones are chemical messengers - they don't supply the actual material needed to, for example, build muscle, but they will change the way your muscles respond. The important point here is that they more often just tell the body when it's ready to go through some development. They're more about balance than just making you feel/develop 'more' of something the more of that hormone you have. Testosterone does trigger penis growth at puberty, and for enlarged clits/trans-men, it's a similar trigger. There's an optimal level you have for growth, and the human body is very good at finding that level naturally. A good example is gynecomastia (male breasts), which has two big hormonal causes: low testosterone, or high testosterone.
The testicular atrophy caused by anabolic steroids/testosterone supplements is your body's natural response, because it's trying so hard to maintain a certain level. If it finds more extra testosterone has been added to your body, it will simply produce less of its own.
The trigger for penile growth is also why it's sometimes used to treat micropenis, as that may be caused by the penis not growing during puberty (often due to a hormonal disorder or testosterone insensitivity.) The growth can be triggered with synthetic hormones, though it's fairly risky and has been linked to increased cancer cell growth too.
Also, you may think that checking a person's testosterone levels is as simple as seeing if they're hairy, bald, and have a deep voice, but that really doesn't work. Those are signs that someone has had enough testosterone to go through those changes, but again it doesn't mean that more hair/lower voice = more testosterone.
The main thing testosterone levels are linked to in adults (women and men) is sex drive, and in that case your T-level is really influenced by other factors that tell your body it's ready to have more sex. By far the two biggest ways to boost this are:
- maintain a healthy weight
- exercise