I wanna know becuase this could change how the game is played and result in more people buying skins just for extra damage and a little advantage in the game.
For the most part, no, there aren't any real changes to in-game stats from skins. However, certain skins have hidden passives that cause extremely minor changes, like characters receiving one less damage from Leona's Sunlight ability if they're wearing sunglasses. A list of hidden passives can be found here.
No, but there's the animation that can change the hitbox visibility, like Ezreal's Puslfire Skin makes his Q ability easier* to dodge.
You can look at this for a more detailed information about this: