Title says it all. I read somewhere that Arceus dragged the player from the future into the time period of LA so is that true? And did Arceus seriously just drag a ten year old from thousands of years into the future all the way to the past and was like 'yeah, deal with this crap kid' before becoming the final boss or something? And I know that the other three become some sort of boss battles but what exactly is the story about how they became boss battles in the first place?
The protagonist is 15 and Arceus pulled the player back in time to create the first Pokédex. At the end Arceus is the final boss because it wants to test the player's strength. Palkia is fought because it wants to help the player quell Dialga or vice versa. Dialga/Palkia are fought because they have become frenzied because Arceus pulled the player back in time and space therefore creating the red sky. Finally Giratina is fought because Volo thought with Giratina's help and all the plates he could capture Arceus and create a new world but destroy Hisui.