how do we have the ending 10 and 15 ?
Currently there are only 14 replayable endings available; we're working on catching up on assets for the latest updates, and once we have those done, we'll be incorporating more fully complete routes (15 and beyond).
For instructions on how to complete the routes, you can refer to the guides on the OYDO wiki site—please note, the story routes page for the site is going to get a major update released on April 15, and more guides will be made available then!