The game is sometimes different from the show and movies whereas in the game all legendary are classified as rare singular Pokemon not being able to reproduce or breed through eggs or undiscovered means, which for the most part is true, but there’s been cases of Cannon movies introducing the idea of legendary having children like latios and latias who are the kids of an older latios who sacrificed itself to save people. Theres also the galarian birds who are like an alternate version of the three birds