So this Chosen One is actually picked out of a line up. That or he was just picked for some reason. For example Po from "Kung Fu Panda" and well it normally doesn't go well. With Po, he was chosen ( many of which believed to be an accident) and lacked any type of fighting skill. Now he does prove he is capable of learning but every one was all like "We're leaving the fate of this land in the hands of an idiot...we're all going to die."
Now when it comes to the chosen Chosen One, there are many things to consider. One is the who picked him ( yes the Chosen One is 6 times out of 10 male.)is the wisest among them, so when they pick someone, they're normally right.
Another method, which actually is pretty random, is "Magic item chose me". One well known example is that of the Green Lantern in DC Comics. A ring will choose a successor, and it is random because a planet was chosen. Now when Kyle Rayner got started, the last Guardian, Ganthet, found him at random in an alley behind a bar and said "you'll do". Kyle did not possess the normal qualities of a Green Lantern (such as being fearless) and had no training with the Corps. Eventually he figures it out on his own and becomes a worthy successor, even playing a key role in bringing back the Guardians and the Corps and joining the Justice League.
Now there are two sub tropes to this Chosen One trope and that's the Poorly Chosen One and the UnChosen one.
Now going back to Kung Fu Panda, there's Tai Lung. Now he trained his ass off to be chosen as the next Dragon Warrior, but it gotten to the point he felt entitled to it. And when Ugwa didn't think he made the cut, Tai Lung was pissed. Now the Poorly Chosen isn't necessarily an asshole, they just turn out like that. Now they are normally rold stuff like "You're destined for greatness," or something like that and when the time comes for such an important task they get over looked for someone else.
And the UnChosen One is the ultimate perpetrator of "Screw Destiny". This is the hero or heroine who stands up, not because of a prophecy, but because they feel the need or desire to stop the Big Bad (sometimes doing so in spite of a prophecy). The Unchosen One is, in essence, a Chosen One who chooses themselves. Like Sam in The Lord of the Ring. Even though the Ring chose Frodo,Sam stuck by his side( he even carried the Ring when Frodo was captured and was able to resist the pull of it) and eventually saves Middle Earth.
Another example of this is Link from The Legend of Zelda: The Wind Waker was not chosen by the gods to be "The Hero". He's not even a reincarnation or descendant of the original Link like every other Link in the series is. He started his quest solely to rescue his little sister at first, then accepted the quest as a whole later. In doing so, re-empowering the Master Sword after it was weakened, and reassembling the Triforce of Courage, the gods eventually acknowledged Link as the "Hero of Winds."
Now a more recent example is Moana. The Ocean chose her to get Maui and restore Te Fiti. But a lot of bad things happen along the way, one being that Moana has this extrastincial crisis and begged the ocean to choose someone else. Her grandmother comes with her ancestors, give her a pep talk and she again picks up the task. The ocean knew that Te Ka needed compassion in order to save the dying world and just assumed Moana was the girl for the job, after seeing her protect a baby turtle when she was a toddler instead of getting the pretty seashell.
There are other examples of this, but I want to keep this under 700 words.