Three of the four mob farm cells at Iron Island already work. This stream is for the fourth and last one. Basket heads down with a load of ocean monument blocks and a worry that there won't be enough of them. johnfal_92 joins partway through and helps with that.
Under the sea floor
Blocks are already riding up the elevator on the way down, which Basket puts down to someone else's digging: "there must just be a backlog." The new cell needs a path in, so Basket builds one out from a ladder and starts carving.
This corner of the farm runs right up against the ocean. The surface is "like right up there," and water pours in as the walls go up. Basket plugs the leaks with cobblestone for now, to clean up later from the top. The prismarine comes from a guardian farm, and "it actually takes a while of AFKing to actually get these blocks."
The tour
Basket stops to give a tour. The corners hold a quad iron golem farm. Between them sit four passive mob farm cells, and this is the fourth. Everything that dies falls into hoppers, converges in the middle and rides up to the sorting room. Slimes might get added later. It's a side project, since the main build on the server is Basketopia, and the prismarine is there "because I rarely use them otherwise."
Running short, and a delivery
To save blocks, Basket makes the inside pads hollow instead of solid. Each corner gets a 2x2 of water, "so it's an infinite spring." Then johnfal_92 joins the game, says in chat they have a lot of blocks for Basket, and drops them off. "<johnfal_92> here." Basket finds them in the shulker box: "You put a whole bunch in there, cool." johnfal_92 adds "<johnfal_92> reserves :)" and later logs off. A few minutes on, Basket checks the pile: "looks like we have all the blocks we need now."
How the cells work
Basket explains it while digging out the floor under the new cell, fast thanks to a Haste beacon. Mobs spawn in the dark cell. They think the open trapdoors around the edge are solid, wander onto them, ride the water off the side and fall far enough to die. Hoppers below carry the drops to the center. Witches don't always die, and neither do zombies that spawn wearing feather falling boots. The prize is gunpowder for elytra rockets. There's a new problem, too: gravel left down below gives mobs somewhere to spawn, "probably a problem later."
The drop point
Up top, Basket turns chests into hoppers ("never have enough hoppers") and grabs furnaces. At the bottom goes a 4x4 of hoppers under the 2x2 hole, a little wider in case mobs stray as they fall, with furnaces on top to cut down on hopper lag. A ring of blocks above keeps falling mobs over the right spot. Basket runs out of sea lanterns partway along, but "we don't need these all to be here to be functional."
Trapdoors, and the first drops
Last come the trapdoors around the whole cell. Basket crafts a few more, opens them all, pulls out the torches and seals the way in with prismarine. Then the wait. Iron goes up first, from the golem farms. Then a mob drops and dies, a skeleton follows, and finally the right item heads up the elevator. "There we go, there's some zombie flesh."
Some styling still needs more prismarine, and a few spots are meant to become glass. For now the fourth cell works. The video ends on the credits, with no gameplay after them.