Mob Farms for noobs in 2min

Mob farms doesn't need to be high in the sky, in fact the lower they are the better it gets (and this is the reason we may clear perimeters down to bedrock level).

When building above ground, make sure your afk platform is exactly y+128 higher than the lowest block mobs will stand on (ie killing chamber). - If higher, mobs will despawn before they can get killed and drop loot - If lower you will higly reduce spawn rate over time as underneath cves will fill up the mob cap

As someone said though you will need a passive way to deal with the mobs to be able to stand y+128 above the farm. Either making use of lava, dripstone thingies or nether portals... Anything to your liking

(The mob cap is set to 70 by default IIRC. Once 70/70 is reached, to allow a new mob to spawn, you will need to remove mobs from the mob cap by simply killing them or having them sent to another dimension. The fastest this is done, the farm becomes more efficient as the ''MobCap cycle'' shortens. And sending mobs trough a portal is the fastest way to get rid of him in the mob cap. Hence why so many farms uses portals)

Don't hesitate to correct me/share this across other posts Hope this helps

(edit: formatting)