Obsidian towers, ender dragon and end return portals generate only in The End biome, causing them to not generate if the biome distribution in the End dimension was changed

I messed around with customized world and then, when I changed the biome distribution in the End, I saw only one tower. When I tried to change the distribution, it changed (not a bug) and did generate without towers.

With the nether one, you'll find that the dragon and the return portal is nowhere nearby.