what the hell is this?

It cant just be marketing from Mojang, can someone explain what this is because I don't understand anything?

Guys, I'm not using mods or anything, I'm playing the game's standard vanilla Minecraft and I just created a new world and these strange structures appeared, I'm confused as hell and scared, I started seeing strange things in it when I walked through the world I created, someone help me aaaaaa I even deleted the map right after that because I couldn't play it