How to optimise my assets for different mobile resolutions.

I searched a lot and asked in the forums but haven't yet found an answer for this question. I'm about to release my first game using SpriteKit, but I have been messing around with unity for a while and I have decided to use it for my next 2D game, but the problem that I don't know how to solve is how to make my game look the same on different mobile resolutions giving that I intend to release on iPhone, iPad, Android. In SpriteKit you could use different assets 1X, 2X, 3X to target different screen resolutions, so what is the equivalent for that in unity? considering that I'm willing to make different assets to cover most resolutions.