지

"right?" or thinking out loud

Ends a sentence to lean on something the speaker treats as already shared: 오늘 생일이지? is "it's your birthday today, right?" and 당연하지 is "of course it is". Turned inward with a question word it becomes the sound of someone puzzling to themselves — 이거 왜 이렇게 맛있지? is "why is this so good?", asked out loud but not really to anyone. That self-questioning use is extremely common on camera. It is casual on its own; adding 요 makes it polite.

Register: casual unless 요 is added