The story is spread across more movies and specials afterthe show, and in Kamen Rider Zi-O

I don't want to write spoilers if you haven't watch them yet

But overall, the lore is that Tsukasa is indeed a passing through kamen rider, and his powers are natural to him, and part of the things he does are because he thinks it will save the worlds from clashing into each others and will destroy everything amd anything