Erich the Fuzzy
Explorer
As 77IM described very well, the DMG has rules for providing stats for objects, and magic items are a class of object. The only thing different about them is that they are noted to be more resilient than a regular object of that type, giving them resistance to damage (all kinds, by default). So you would stat it up as a regular object for AC and hp, then any damage to it would be halved. (If you like, you could add an appropriate immunity or weakness, as 77IM suggests).
Where I would differ somewhat is on allowing a monster to target the item to damage it, or (by implication) allow PCs to target items carried by monsters/NPCs, other than a few possible situations where an attack or effect is specifically written as targeting or affecting objects (generally, corrosion effects from monsters) being carried or used. The standard for 5e is that an item (object) being worn or carried cannot be targeted or damaged by a regular melee or ranged (weapon or spell) attack, and is not affected by area of effect damage. Instead, the target creature is affected (or not, on a miss or save). In some cases (like disintegrate, or petrification), if the target dies, their items are also destroyed (or petrified), but if they survive, their equipment is fine. largely because surviving, in the D&D hit point system, means little or no actual physical damage was dealt. Basically, you can't damage or destroy an item/object being held/carried (by implication, worn) unless you first separate it from or kill the possessor.
5e does have a few specific mechanics to separate something from the creature that is carrying/holding it.
The first is a sleight-of-hand attack to pick a pocket; this would not work on something being worn (other than a hat) or held. So you could try to grab something from a pocket, backpack, scabbard, belt, or snatch a hat or possibly accessible jewelry (like a necklace, amulet, circlet, or brooch). The DMG suggests this is opposed by the target's passive wisdom. I might allow an attempt to grab an object being held using this skill, but have it opposed by an active STR or DEX check (with advantage, if held in both hands). I would also give advantage to all attacks against the character making the attempt until their next turn, because they are busy with a tug-of-war rather than normal dodging. (Similar to the Reckless effects).
The second is an attack to disarm. DMG chapter 9 has rules for attacking (vs. Athletics or Acrobatics) to disarm a person of what they are holding, forcing them to drop it. The Battle Master can also use a Disarming Strike, which has different mechanics but the same result for the object - it is dropped. Both of these require a weapon (melee or ranged) attack. (So the theatric "shoot the gun out of his hand" trope is RAW.)
Once the object is no longer being worn/carried, it can be directly attacked and damaged and thusly destroyed. Or you can use a free interaction to pick up the dropped object, then for good measure, move away with it (if you have any move left).
In the case of the halfling on the broom/carpet and the giant, the giant could throw a boulder or other object (or swing a greatsword, if within range) to make a disarm attack. If it succeeds, you could say the broom/carpet has been knocked out from under them, or they were knocked off it. No attack damage, but falling damage is likely. The giant (or some other creature) could then go to and pick up the object. In the case of a broom or carpet, I'd have the giant pick it up and break/tear it in half. If it's too far to pick up, it could be targeted directly by a weapon attack - a thrown boulder for the broom, or swung greatsword or a lightning bolt for either item. A creature with an appropriate multiattack could do a disarm and a destroy in the same attack sequence, otherwise it will take at least 2 turns' actions to get rid of a pesky magic item. So it's not fast (as targeting an object directly for damage would be), or automatic (having area of effect damage also harm a target's possessions), but it can be done if that's what the creature wants/needs to do (and can roll high enough).
My preference is always to utilize rules already in the manual (for disarming items, and for targeting/damaging objects) than to make up new rules that may not be balanced or might have other unexpected effects on play. Especially when existing rules give a clear and reasonable path for achieving the creature's goal.
