I always looked at Kyurem as being similar to how Shedinja is created.
When Nincada evolves, its leftover exoskeleton gains life as Shedinja. I just applied similar logic to Reshiram/Zekrom splitting apart. Kyurem is the discarded shell that gained a will of its own. --- However history remembers me, it shall only remember a fraction of the truth.
What Arceus all created has always been a rough point. It's only really been stated to possibly have created the Lake Guardians and the Dragon Trio, as well as Sinnoh. The specifics of what it created really are mysterious, since the Pokedex said that Sinnoh had a myth that it made the universe with a thousand arms, while most of the other lore said it just made Sinnoh and a few select Pokemon from eggs.
The site got Palkia and Dialga's roles mixed up, and the names of the Kamis are wrong (in English, at least, so the site must be really old). --- The Creepin' Squid In search of new life... to loot