Folia is probably the most interesting on this list - it's a mod by the makers of PaperMC to parallelize Java Edition while still maintaining a working survival experience. It's best known for powering 2b2t, a anarchy server with a current player count of 633 on a single survival world, with the world being roughly as old as Minecraft multiplayer itself (servers that are effectively multiple servers chained together have hit CCUs of 216k).