like generating a town map as desired, or calculating the time when a certain bug/fish will spawn

Thank you @GZ! That completely answers my question. I'm not yet sure how to do RNG manipulation, but I'll share it when it takes shape to a certain extent.

Actually, I used something like RNG manipulation while doing golden rod speedrun. ( 4:06:15~) A viewer suggested the dates and I followed them, then the fish that I wanted were spawned. I don't know how to do that, but probably it was a kind of RNG manipulation. It was unprecedented, so I can't judge whether it's ok or not for submitting run on SRC. This is one of the reasons why I created this thread. (of course I have to ask moderators for a final decision)