Want to improve your grinders? Here's some tips to do so!
Now, in the end of the day we are playing Minecraft, which allows endless amounts of possibilities for any problem you might experience, this guide will hopefully provide you with several tips and points that will help you improve your grinder and give you desirable results.
One of the first things you might want to improve in your grinder, is being able to control what mobs spawn and what mobs don't spawn, here's a few tips to remember:
Spiders need a 3x1x3 flat area to allow them to spawn, therefore if you follow the pattern to the right with trapdoors, you'll be able to completely block spiders from spawning in your grinders.
No dolphins to see here
You can cover the roof with trap doors as well to prevent any mobs from spawning that are taller than 1.8 blocks, meaning only spiders and creepers will spawn now, you can mix this point with the previous point and make it so that only creepers spawn.
Alternatively, if you're in the nether, you can use blocks that only allow a certain mob to spawn on top of them, for example magma blocks only allow zombie pigmen, ghasts, and magma cubes to spawn on top of them, however if you only allow a 2 block gap above them, then it prevents ghasts and magma cubes from spawning, and only pigmen.
Similarly, wither skeletons are the only mob that can spawn on wither roses for being immune to the wither status effect, you can use this to your advantage if you cannot afford a wither skeleton spawner.
You can obtain wither roses by allowing a wither to kill a mob,
As mentioned previously, some mobs are naturally aggressive towards certain other mobs/blocks, which you can use to lead the mobs into a drop and trap them for killing:
Iron golems attack most hostile mobs.
Zombies attack Villagers.
Illagers (Evoker, Pillager, Ravager, Vindicator) attack Villagers.
Endermen attack Endermites.
Piglins AND Wither skeletons chase each other to kill each other.
Wither skeletons attack baby turtles, iron golems, snow golems, piglins, and piglin brutes.
Zombie variants and blazes attack turtle eggs.
Undead mobs (Except zombie horses and skeleton horses) attack baby turtles.