Genshin Impact: Terraria Begins

[The writing is pretty average, no single heroine, not much depth. Just read it for fun. Harem story, novice writing, characterizations change. Best not to read.] After transmigrating to Teyvat and arriving in Liyue, why did I find creatures that belonged exclusively to Terraria? Good heavens, giant eyeballs flying in the sky? A huge brain that can split into several? Sumeru has been taken over by three Jungle Kings? Mondstadt is corrupted? Fontaine has been blitzed by a goblin army?