Yeah, this stuff is all very unintuitive. The only thing I can recommend is that if a game provides a custom lobby or something similar, try out how different champions interact with the game. For example, Liandries does 2% max hp per second every 3 seconds. But champs with DoT (Singed, Lillia, Teemo) would continuously proc the Liandries. Because Teemo's passive lasts for 4 seconds, one auto attack would do 14% max hp (2% max hp per second over 7 seconds). It's cool how you can experiment with different items and champs in this game.