Testosterone makes it easier to build muscle mass, but it means you still need to use your muscles to build it. Should be noted.
As someone who went from an unfit slime blob (I still have a lot of body fat though, so I still look like a slime blob) and to being very fit today after I began working out while on T, T helps in this regard. I did build some muscle doing nothing but I wasn't of course doing nothing per se. You use your muscles for everything.
