Donning or Doffing a shield requires an action
Donning or doffing armor is separate from the "Use an Object" action. Notably, a shield is the only kind of "armor" that takes 1 action to don or doff. The rules for donning and doffing are pretty clear:
Don. This is the time it takes to put on armor. You benefit from the armor's AC only if you take the full time to don the suit of armor.
Doff. This is the time it takes to take off armor. If you have help, reduce this time by half.
It doesn't name any kind of action, it just says that this is how long it takes to don/doff the armor/shield. Notably, it stresses that you only benefit from the armor if you take the full time to don it. Donning or doffing a shield is mechanically the same as donning or doffing armor, with the only difference being the time required to do so. The fact that the don/doff time for a shield happens to be the same as the time required for Use an Object doesn't make it an instance of Use an Object.
Donning/doffing a shield is not "using" it
You use a shield by putting it between you and an incoming attack. Mechanically, the shield is automatically used every time something rolls an attack against you, and this use is represented by the static +2 bonus to AC. You can only use the shield in this way if you are wearing it (i.e. you have spent an action donning it). No action is required to use the shield in this way, so the "Use an Object" rules do not apply.
There are some features that give you ways to use a shield with your action, bonus action, or reaction, such as the Shield Master feat or the fighter's Protection fighting style. However, all of these are special actions on their own, not "Use an Object".
If you wanted to improvise the use of a shield to do something like using it as a lever to pry open a stuck door, that would be an instance of "Use an Object", since it is not covered by any of the specific mechanics of shield use.
Magic shields are no different, unless they say so
Donning or doffing a shield is not considered activating a magic item. The only time you can activate a magic item is when the item's description describes an action you can take to activate it, as described in the chapter on magic items (emphasis added):
Activating some magic items requires a user to do something special, such as holding the item and uttering a command word. The description of each item category or individual item details how an item is activated.
A standard +1 magic shield doesn't describe any way of activating it, so the rules for activating magic items have no bearing on it.