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:
There are actually some skins that have advantages. For example iBlitzcrank is banned in pro matches just as Heartseeker Ashe, due to their abilities being harder to see. This would lead into giving the players an unfair advantage.
Yes, some skins have this kind of effects, but it's easter eggs. For example all skins with sunglasses take 1 damage less from Leona's ultimate (protection against sun rays) Theese easter eggs are the only admitted bonus a skin can grant ingame.