@boag Isekai literally translates to "different world". The concept of isekai in a novel/manga/anime is that of an MC doing his stuff in this "other world" after being summoned there, reincarnated there as a baby or 'something' happens and they transmigrate there. Summoning, reincarnating and transmigrating are the only way for a story to be called "isekai". If the MC was born (first-time) in that world, its a regular Fantasy setting, even if the MC is technically reincarnated as seen with Anos Voldigoad in Maou Gakuin.
Maou Gakuen = Fantasy
Danmachi = Fantasy
Re-zero = Isekai
Konosuba = Isekai
Maou Gakuen = Fantasy
Danmachi = Fantasy
Re-zero = Isekai
Konosuba = Isekai