In this case, nothing. High dose testosterone is a hormone, hgh is a hormone. Both are PEDs (performance enhancing drugs). Now, the difference between them is a bit more interesting.
Testosterone, the original steroid, makes you big by maximizing your existing muscles (super paraphrased, as is everything I’m about to say). It’s the one that gives you breasts and shrinks your balls (for those that have them).
Human growth hormone makes you big by inducing the creation of new muscle. As well as everything else. The stand outs being the heart, which you really don’t want to grow, and the intestines, which gave bodybuilders roidgut.
Like I said, very paraphrased, but that’s the gist of it. And doesn’t touch more advanced things like tren.