The library from the last few episodes still has dirt where its books should be. Real bookshelves take books, books take paper, and Basket has been getting them by trading sugar cane. The sugar cane farm on hand is a prototype that doesn't produce much. So this stream goes back to Iron Island, where the underground iron golem farm already is, to make room for something bigger.
A bigger plan for Iron Island
The plan is to send the farm's iron up to a room near the top, put a sugar cane farm in the space around it, and maybe add a spot where mobs spawn, die on their own and drop their loot. Basket admits that last part wasn't planned: "I was clearing out the area and then all of a sudden I walked away, came back and there's tons of mobs sitting there." The main thing wanted from it is gunpowder.
johnfal_92 is on the island too, by nameplate, and drops a line in game chat: "need anymore slime." A second one follows: "another stock of slime delivered." Basket shows off the AFK room down among the storage chests, where everything goes into a chest before AFKing in case something goes wrong, and says the AFK spot will move upstairs once the iron does.
Doing the math
The spawning floor has to sit far enough from the AFK spot that mobs spawn there and wander off on their own, but not so far that they despawn. Basket looks it up, settles on 30 blocks down, counts every one out loud, then moves it three blocks higher to be safe. "Okay, so John, if you feel like you want to verify my math," Basket says, and goes ahead with it anyway.
The design is the same idea as the Team B.S. record shop: mobs spawn on pads, walk off the edge, get washed by water to a drop in the middle, and fall to their deaths near an item elevator. Basket isn't sure the drop is tall enough yet and floats setting them on fire partway down.
Pads, slabs and trapdoors
Basket digs out a test area and builds 3x3 pads. Each gets a slab in the middle so spiders can't spawn ("they're just a pain to work with") and wooden trapdoors around the edge. Closed trapdoors look like walkable floor to a mob, so it tries to walk onto them and goes over the side. Making the trapdoors means chopping wood, and later a lot more of it.
Water won't behave
The water is where it goes sideways. Basket tries stepping the channels down a level at a time so each stream ends right at the edge, counts blocks, and watches the water spill over anyway. "Huh, I thought that would line up right." The fallback is signs: placed in the air, they stop water exactly where needed with no counting. "It's not ideal." Basket raises all the pads one block for more room to work with, which means pulling up trapdoors and putting them back.
By the end there are four signs per section and still not enough of them. Standing in each stream to test it turns up dead spots where a mob would stop moving and clog the farm.
Back to planning
The stream runs out before the farm works. Basket walks through how it should go (spawn, walk off, wash down, fall), then admits it doesn't yet. "For now this is not functional." The pads and the trapdoors stay; the water layout needs a rethink before the next try. The video ends on the credits, with no gameplay after them.