The changes in the appearance is down to the masculinizing traits of testosterone. The more test you take, the higher levels of androgens you have.
This will in turn affect the physical traits of the body. Your body will change according to testosterone levels as testosterone determines these physical traits.
That's why when you get someone who has had low T all their life and get them on a TRT cycle, they completely transform. It doesn't matter what stage you are at in life, your body will change according the presence of sex hormones; positively or negatively. There is a reason why it's so highly sought after. This sh*t works.
Also, some of the benefits you may be seeing with facial complexion will be down to collagen, which is increased through the effects of testosterone. Your body is designed to respond to sex hormones, androgen being one of them. Collagen is very important for lots of things in the body; hair being a big one, bones etc. Skin obviously. The beauty industry puts collagen into their products because it's widely known to give that 'plump' effect in the skin. You buy it as a supplement.
Then you have sebum; skin oil.
More skin oil means more moisture. Moisture hydrates the skin. This oil contains stuff our skin needs. Too much and we have really oily skin, which is something that is very common with people who take steroids. We have skin oil for a reason and just because too much is a bad thing doesn't mean having it at all is bad. It's actually good.
Linked to this, you also have acidity levels in the skin. The more test levels you have, the more acidicity levels your skin has. This affects your skins ability to maintain moisture but also helps to maintain the skins natural ecosystem; including warding off infection.
Also, you have blood circulation. Testosterone is well known to affect blood circulation. The more test, the more effectively nutrients are shuttled around the body. This goes for why most people take gear in the first place; to greatly aid in protein synthesis, which in turn improves recovery and therefore faster muscle growth. Muscle growth is not the only thing that is affected. Also every other organ in your body, including the skin, benefits from this enhanced circulatory effect. The advantages of this is your body can send things around it to where they need to go. If this is improved, the overall operating of the body is improved along with it's vital systems. Again, why TRT is so beneficial, to those who need it. It can fire up systems that were not firing up properly before.
You have to balance everything out though. TRT is not a panacea and what most people consider TRT is actually a low dose steroid cycle. This isn't necessarily therapeutic.
The risks increase the higher up the dosage chart you go. Ideally you want to be somewhere around natural levels, if not a touch above, to benefit from enhanced results, but nothing higher than that otherwise you risk tipping the scales in the direction of potentially negative side effects.
Balancing this out with a healthy lifestyle and active involvement in looking after yourself is key. Test won't do this on it's own.