Start with Type-Moon, but Magical Index Magician

Morpheus, having traveled to the world of "A Certain Magical Index," was still sorting out the mess left by an evil magic society when he was caught in a space-time turbulence and transported to the world of Type-Moon. He was reborn as a baby, adopted by the Macremis family and Bazette's younger brother. As he prepared to fight the Fifth Holy Grail War, he discovered that this world was actually the FGO universe, where the Fifth War had never existed. Thus, for the sake of world peace and to once again pursue the ultimate secrets of magic and achieve the path of a demon god, Morpheus Fraga Macremis, a golden magician, embarked on a journey to save humanity.……The introduction is powerless, harem, infinite flow, the current world is fgo, and the target worlds include Genshin Impact, Hokkaido, and Demon Slayer are also under consideration.