Watch youtube. Turtorials there.

some more specifics:

build the outside dimensions of the box 20x20. then inside build four 8x8 platforms with a channel two blocks wide between the platforms.

in these channels put water sources at the edges. the water will naturally flow towards the center and stop just at the edge of a 2x2 area.

this 2x2 area is the drop chamber. dig this down ~25 blocks.

the mobs spawn on the 8x8 platforms and walk into the water streams that carry them to the drop.

you can make as many vertical layers as you like although you will start getting diminishing returns after a certain point.

people say you need to build it in the sky, but mine is at ground level with the kill floor ~25 blocks underground. it might be more efficient if it were up in the sky and if i AFK'd exactly where the math says i should be AFK. but honestly i just build one at ground level in my base and then just go about my day, it will slowly but surely fill up with mob drops.

my latest fresh start i built one as described above, 20x20 with four 8x8 platforms, at ground level. i never "AFKed" at the farm but may have been AFKing at a nearby AFK fish farm. it provided me with enough arrows to eventually kill the ender dragon.

You need to make a big closed off box in the sky that the mobs can spawn in. In the middle of box make a cross or a plus going from one end to the other. Fill with water overlapping a hole in the middle. Cover the middle with signs so the water goes over the signs but does not go below them. Then you have to figure out how far the mobs can fall and have half a heart left. Put the box so that they fall that height. Make sure they can't get out from the bottom and just kill them.

Water ->>> water elevator->>> kill chamber

Dig out a flat space 2 blocks below it, and have it go 8 blocks in one direction, and at the end of that build at drop off area of about 12 ish blocks (I think it can go to like 23 or something though). Then put water at the opposite end of the drop. If done successfully, whenever a mob spawns, the water will push them down the hole, the fall will weaken then, then you can make a hole to hit the mobs through. This was hard to put into writing, so if you have any more questions, feel free to ask.

Use ilmango’s design. It gets the most out of spawners since they’re only good early-game

https://www.reddit.com/r/Minecraft/comments/9mr3rm/minecraft_skeletonzombie_xp_farm_113_aquatic/?utm_source=share&utm_medium=ios_app

this is the one that I’m currently using. It’s pretty efficient. I’m not entirely sure if it’s the best, but it’s pretty damn good

I built one and think I might be too close to the river "biome" since I'm in taiga. I also think ill need to test in hard mode.

That’s cause it’s a pretty bad design. Build one that uses water flushing the mobs off the platforms and build it above an ocean or desert. Then pillar up till you’re just under 128 blocks above the lowest point of the farm to afk

I think they can’t spawn in water

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A wolf at the bottom of a 23 block hole I think and let the wolf kill them but it has to be taimed idk if you can do it if it is sitting so it does not teleport to you but creepers would probably be a no no bc they might blow up and kill the wolf idk about the mobs and piston thing also I think it counts as a player killing it so you get xp

you can use a magma block as the floor block of the hole. this will kill them eventually.

Campfires or magma blocks will kill mobs and the items will just be dropped, then you can use a hopper minecart to pick them

Try soul camfires

Use a trident killer if that works on java

You need to specify Java Edition or Bedrock Edition, since the mechanics are different.

There are countless tutorials, are you looking for pure XP or drops

Build it high over an ocean

Build a fence, bring cows/sheep/chickens/pigs and breed them

Sky limit is 100% better.

The y level is irrelevant for spawns.

As for a better design than Etho's old school mobavator, I made a multi-level version without dead spots (they could still get stuck, but only very briefly) on an SMP server I played on back in the day. Unfortunately between me and the old admin, we both had hard drives crash over the years and have lost the map and I don't recall exactly how I did the water flow on the lead-in to the mobavator, but it was pretty complex and took a lot of experimenting. I kind of had to do a multi-level version in order to get enough spawn spots for it to work well with other players elsewhere on the server; SMP has a shared mob cap across the entire server.