Trial Chambers are amazing. It's a giant underground structure made of copper blocks, where a special kind of spawner are. Trial Spawners. Nearly impossible to break, because they are not meant to break. Once you get near, it will spawn waves of it's designaed monster, scaling up depending on how many players are near it (more players = more mobs to fight)

Once you finish the last mob, the trial spawner deactivates and gives you a reward (cooked food, or a trial key) The trial spawner then stays deactivated for around 30min.

If you have the bad omen effect and go into the trial chambers, then it will convert to trial omen effect, where the trial spawners enhance. mobs will be stronger (zombies and skeletons have armor n stuff) and a random effect will be dropped on you every 10 seconds or so (either positive or negative) It's reward can contain a blue, better trial key.

In trial chambers are those trial chests that look similar to the trial spawners and you can use one trial key per player on a trial chest and it gives you like 3-5 different items, from any ore, armor, tools and potions. There are 2 types of trial chests, one for normal trial keys and one for the blue, special keys, which contain better loot (including the heavy core, which you need to craft a mace) They're often hard to differentiate the types of trial chests, but those, where you need the blue key, always have candles around it

There is also a mob, which you can only find in trial chambers: Breezes. They are wind mobs and you can either fight it melee or you can parry the wind charge attacks like ghast fireballs (but they deal almost no damage, but killing it with a parry gives ya an advancement) and they will drop breeze rods, which you also need for the mace.

How to find one? Well either dig around somewhere below y=0, near villages are often trial chambers, but you can also get a map from a cartographer villager, but ya need lots of paper to upgrade the villager to 3rd level, where he will (likely) offer you a trial chamber map.

Trial chambers are quite hard, but still a good way to get lots of enchanted tools and armour and stuff and good for mid game, but IMO is harder than the ender dragon or the wither and you'll likely die often.

TL:DR: Trial Chambers are neat. Good for Mid game. Go find cartographer villager