Many Classical sculptures depict men with impressive muscular bodies, even by modern standards. Who were the models for these statues? How did they achieve these physiques?

This question is common enough to make it into the "very frequent" section of the FAQ, which links to this thread with an answer from u/Iphikrates and links a bunch more answers to similar questions. There have probably been more in the meantime since that thread which was 5 years ago, and hopefully still more to add!