In the PHB, on page 193, the Use an Object action is described:
Use an Object
You normally interact with an object while doing something else, such as when you draw a sword as part of an attack. When an object requires your action for its use, you take the Use an Object action. This action is also useful when you want to interact with more than one object on your turn
But what exactly is considered an object?
More specifically:
Is an arrow an object? Are weapons, armors, and shields objects?
An object is any physical thing that is not a creature:
... targets creatures, objects, or a point of origin for an area of effect...
Chapter 10. Spellcasting, pg. 204 of PHB
The above description lists creatures, objects, and points in space as the 3 distinct, mutually exclusive types of targets. Note that spell effects are not physical things, and so are not objects, creatures, or points in space, though spell effects can affect, create, or share space with one or more of them.
A creature is anything that can take any (or at least most) of the general actions. All creatures fall into the categories of either Player Character or Monster:
A monster is defined as any creature that can be interacted with and potentially fought and killed... the term also applies to humans, elves, dwarves, and other civilized folk who might be friends or rivals to the player characters.
Introduction, pg. 4 of MM
So, you can also say that anything besides the PCs that isn't covered by the MM is an object.
This means that arrows, weapons, armor, and shields are, in fact, objects. NOTE: using some objects have more specific actions, so the "Use An Object" action you quoted does not apply. This is case for arrows (ammunition property of weapons and the attack action), weapons (the attack action), and armor and shields (donning and doffing and armor class). If you do something unconventional with one of these objects, such as using a sword as a lever to crank a wheel, the "Use An Object" action would apply.
From DMG p. 246 (which, by the way, is where Crawford tells us to look at if we want to see "What counts as an object in D&D") or here in the DM's basic rules:
For the purpose of these rules, an object is a discrete. inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
All the items listed are objects, as an arrow is a discrete inanimate item, as well as a weapon, an armor or a shield.
This definition gets fuzzy when taking into account magical items with sentience, as they are not inanimate any more (but they are still objects), but we do not have a canonical answer then.
I would also like to note that, while this is the game definition of the term, it doesn't seem a good one, and sometimes relying on common sense would be better. As mentioned in the responses to the linked Crawford's tweet, a book consists of many other objects (pages, cover), as well as clockwork toys...
ob·ject
noun
ˈäbjekt
- a material thing that can be seen and touched.
The term "object" has no special meaning in the rules. 5e D&D's rules use plain English and they try not to infer special meaning beyond the regular English definitions of words.
5e D&D's rules are also exception based: a general rule always applies until a more specific rule applies. This can be applied to all objects. Since "object" is a class that encompasses all things, "Use an Object" is the generic action you take when you want to interact with an object that doesn't already have some other rule for interacting with it.
For example, you wouldn't Use an Object to attack with a weapon, because weapons have a more specific action for their use, the Attack action. You could, however, Use an Object to interact with a weapon in a non-attacking manner, such as drawing it from a sheath. To clarify, the rules do specify that you get one free "object interaction" per turn,
You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.
which, as stated, can be used to draw one weapon as part of a move or attack action, but if you were to draw a second weapon in the same turn, that would require the use of the Use an Object action.
Note that this only applies to the generic Use an Object action, and is limited to things you could reasonably do in "less" amount of time than a regular action. Drawing a weapon or opening a door while you move is reasonable. Tying a rope around something is probably not. This will likely vary from DM to DM.
Using a magic item, similarly, has its own rules associated with it and does not normally use the Use an Object action.
Arrows, weapons, armors, and shields are all objects, but if you read the rules for these specific types of objects you will find they all have specific rules for how they are used. To "use" armor (that is, to wear it), you must take time to put it on, and that time varies from armor type to armor type.
Shields are actually a type of armor, and all armors have rules for donning and doffing: it takes one action to don a shield, and 10 minutes to don heavy armor. Donning armor is not Use an Object because it has a specific rule on how to use it.
Arrows have their own rules as well:
Ammunition: ... Drawing the ammunition from a quiver, case, or other container is part of the attack (you need a free hand to load a one-handed weapon)...
So when you use an Arrow, you're doing so for free as part of your attack (unless you're not using it to attack with, in which case, you guessed it, it's Use an Object).
All this to say that the Use an Object action is a catch-all that should be used generically when no other rule or action can be taken to interact with or otherwise use an object.
If you want to do something with them that (a) isn't part of another action, (b) isn't an attack, or (c) other rules say requires an action, the you need to take the Use an Object action.
During combat, on your turn, you can interact with one object "for free" as part of your movement or action:
You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.
(PHB, p. 190, emphasis mine)
This covers many cases where arrows and weapons would be "used" in conjunction with an action.
If you want to interact with a second object, you need to use your action.
(PHB, cont'd)
So if you've already drawn your sword as part of your attack action, you can't draw a second weapon to make an off-hand attack on the same turn. Arrows and weapons would again be 'objects' for the purpose of this rule.
Some magic items and other special objects always require an action to use, as stated in their descriptions.
(PHB, cont'd)
Shields, per the Donning and Doffing Armor table on p.146 of the PHB, take an action (the Use an Object action) to put on or take off. Armor of any kind takes much longer, and each round a character is putting on armor, they would take the Use an Object action.
Use an Object doesn't include attacking with arrows or weapons, of course; that's what the Attack action is for.
