This is a re-done version of Burns1970's Mob Farm, which can be found here.
I tried contacting Burns1970 but got no reply :( . So, Burns if you are reading this and object to me posting this, let me know and i'll take it down right away.

It's automated, with 3 levels of spawning monsters that fall down a big vertical shaft onto hoppers that link to chest. That's about it, pretty simple. Put schematic in, get items out. Updated for 1.16.1

Main things I changed from the original:
-The original had 2 of the levels not working because the drop wasn't enough to kill the mobs, so I fixed that
-Added Monster Spawners to speed things up a bit. Feel free to add even more if you want faster or different loot.
-Changed the outer aesthetics somewhat.
-Added Maintenance hatch to the roof so you don't need to fly up there. It can be accessed using a ladder at the back of the building with maintenance gantries
-Bunch of diamond gear on the roof chests for if you do go in whilst there are mobs so you're not unequipped.

Download schematic is the .nbt version (vanilla minecraft structure) which definitely works with litematica and most other things. Alternative download link has both the .nbt and .litematic file in a dropbox, help yourself :)

If you want to use on your server or whatever, that's fine with me, you don't need to message or anything, just go ahead, but a credit on a sign on the building or something is always nice, but not necessary. Also you may wish to check in with Burns1970, as it is their original design that I've modified.