(exogenous) HGH is a injected peptide that imitates the naturally produced HGH by your pituitary gland, consisting of 191 amino. It is used by a lot of athles since it stimulates cell growth and regeneration, which is useful to recover to recover by producing IGF-1 which helps repair tissue which is issue for recovery (muscles are tissue after all)
HGH is one of the most studied petpides on Earth so I can tell you both personal experience and studies.
But basically what it does is it improves bone density, gives height grow for youngcels, gives you better sleep, and increases energy by helping your body use carbs and fat more efficiently for energy. (anti aging effects are cope it's just marketing)
Of course it has it cons even if you use the littlest of doses. Mainly bloating due to effects on sodium and water balance which is a looksmin of course. Cartilage growth is also very common (basically abnormal growth in the nose and ears. The side effects get worse the more you up the dose but apart from those 2 it's safe short term. Long term carries more complications but I'm guessing you're only interested in height.