Tsukihime has the most plot points without descriptions or explanations, like the 6th System that Zepia wants to reach, the Aylesbury ritual, the will of the planet and whole lot of shenanigans that he simply hasn't addressed since 2001, barely mentioning them in character entries, random comments inside the game's files and/or random rants on their blog, with no further explanation about them.
Why?
>>153529977
There's shit in the art books. One of them has a short story about doa's preparing for the ritual. Gaia and Alaya are discussed briefly in the fate/stay night vn. That's all just macguffin shit anyway, can't really explain it better than "magic".