only one principle needs to be known and understood.
SAID. "specific adaptation to imposed demands".
i..e you impose a demand on your body, it adapts specifically to manage that demand,. after it masters that demand, it means that no longer you get that same response. thats why no fixed rep pattern, or diet, or any form of stimulus works- at least optimally, for longer than a few weeks.
and thats also why "mixing up rep schemes regularly" doesnt work either. - because the demands vary so much that the stimulus isnt something that requires a specific adaptation to.
so you end up not responding at all, and any introduction of a new demand is readily dealt with.
the best analogy is learning to juggle. hard at first, then easy...add in another ball...hard again, then easy. try 3 balls instead of 4...not so easy again, and so on. dont juggle for some years and then come back to it...etc etc.
im sure GB members will contort this post as usual and apply their simplistic version of it in order to ridicule it. thats standard here.
doesnt matter, its well known fact based on homeostatis and exists in other forms like all chemical reactions, magnetism, heat transfer, string theory...