Itachi's knowledge came from Obito, whom he was trained by. Obito himself who was trained by Madara.
Just putting this out there, but Itachi was never trained by Obito. After the deal they made, they rarely ever interacted in the Akatsuki and traveled with their respective teammates. Itachi told Sasuke that he was his mentor in order to give an excuse as to how he knew "Madara", because Itachi wanted to tell the story about how Madara acquired EMS. Itachi knew that one day Sasuke would become blind, so he thought that if he told Sasuke that story, then he would take his eyes and gain EMS himself. Itachi just went through the Academy system in Konoha, learned basics from Fugaku, and maybe a few pointers from his best friend Shisui. Then he left Konoha and was on his own as far as training goes. His knowledge came from reading the Uchiha tablets, something he stated himself.
