I'm pretty sure it was a sci-fi novel, and in it the MC goes to a janitor? who is in fact one of the strongest people of the planet, and MC does super intense training with him. I think it was something similar to Emperor of The Cosmos.
If my memory serve right For tempest, I read in the synopsis the mc meet expert whi is a librarian or bookkepeer something like that For 40 millenium, the exoert.that teach him is former refiner Try dark net