How does it know that "card" is one of the numbers in the range of constant "cards"?

It sounds like your talking about the for loop in one of the array challenges, right? Something like this?:

let cards = 1...13

for card in cards {
    //other code here
}

If that's what you're looking at, I understand your confusion. It does seem almost magical how the variable appears out of thin air.

It turns out the for loop defines the card variable for us. It's almost like it's writing var card = cards[0], going through the loop, and then writing var card = cards[1], going through the loop again, etc. up to cards[12] - the last item in the array. We just get to use one convenient statement to achieve all of those effects.

Since it's a variable like any other, we can also use it in the loop. If it helps, remember card could be any correct variable name - like loopCounter or i.

this also confused me. it's not pointed out in the video that "card" doesn't need to be initialized.