Teamfight Tactics In-Game Targetting

Hello there Hivemind!

Got a mechanical question for your; what is targetting?

Having place a double ulting Veiger, gold Vayne, fully glacial Ezreal or double spawning Azir in every line, they all focus the biggest, baddest dude on the field. How the hell do I get around this? Even if position myself on the other side and keep my games safe or line, they still focus the tank, so what gives?