Where do you think testosterone comes from?
The test fairy comes along and injects you at night?
DNA makes RNA which makes protein.
Protein is structural as well as the enzymes, receptors, some hormones in your body.
Testosterone is derived from cholesterol, which is either synthesised in your liver or taken in due to diet (how much of each, determined by genetics), and the various enzymes convert it to test.
The size of things like testicles range significantly in size, due to what? Genetics. More testicles, more testosterone.
Test in converted in the peripheral tissues to DHT. The number of test receptors, genetic, the efficiency of the enzymes that do all this work, genetic.
Other factors that affect muscle hypertrophy, mTOR pathways, genetic, myostatin, genetic, follistatin, genetic.
How your muscles look is determined by insertions, which are guess what, genetic.
This is a gross over-simplification of some of the basic genetic factors.
There is obviously a whole interaction between the genes and the environment, but it is nature via nuture, or the expression of the genes you got influenced by the environment.