A major part of hypertrophy is increasing the number of nuclei in muscle cells via satellite cells. Muscle cells are either one of very few, or possibly the only type of cell to contain multiple nuclei. Anyway, while training, there is a certain amount of “swelling” in the muscles (sarcoplasmic hypertrophy, I think) which will stick around for weeks to even a few months after stopping training I believe. However, the extra nuclei stick around. That’s the reason people who used to be muscular generally have an easier time getting back in shape than someone who is trying to build muscle for the first time. I remember reading about a study in rats where they were given a round of testosterone injections, which understandably increased muscle mass, but even like a year later, they still built muscle more easily than normal. I’d have to search for that again, I forget where I read it.