If you use the attack action as a monk then you will qualify for an additional bonus action unarmed attack. The initiating attack action does not need to be unarmed (though it can be). This bonus action unarmed attack does not cost ki, which makes it useful if you are out, or if you want to save some for later.
Off hand attacks also require a bonus action, so if you have only one bonus action then you must use one or the other. If you take the thief subclass then you will have 2 bonus action attacks, and you could opt to use one for unarmed attack and one for off hand attack (but you would need to use the attack action first, in order to qualify for the bonus action unarmed attack).
I don't know how unarmed attacks interact with sneak attack in BG3, so perhaps somebody else can answer that. I do know (and have personally confirmed) that if your strength is higher than your dex you do use strength to attack and damage for unarmed attacks.
