For randomizer information, see: https://github.com/homothetyhk/RandomizerMod .
These mod are heavily based on work done by many contributors before, see: https://github.com/CallumMoseley/HollowKnight.RandomizerMod
ItemSync is an addon to the randomizer where any randomized item pickup will be shared between all the players in a "room". MultiWorld is an addon to the randomizer where items are not only scattered throughout your file, but through any number of linked game files. When picking up an item that belongs to another player, they receive it in their game. This allows co-operative randomizer playthroughs where you may need to pick up each other's progression to go forward.