basically, if when sheldon from young sheldon was 11, his dad lost his job (again) and his job couldn't support caring for all three kids, he had to put two of them up for adoption, he put sheldon and Missy up for adoption because he didn't want them to suffer being as poor as they were, Missy and Sheldon both got adopted by people in Japan, Missy got adopted by the Ashidos and Sheldon got adopted by Inko Midorya, Sheldon changed his name to Izuku Midorya and Missy changed her name to Suya Ashido. Sheldon and Missy haven't seen eachother since they were 11. (Missy is in the support course and is friends with Mei), Sheldon met Bakugo when they were 12, Izuku doesn't act as smart as he used to act, he's still smart he just uses like, 5-20% of his smarts.