Publishing for example to iPhone will it automatically resize it for the size of the iPhone’s screen including sprites etc?
That’s a very vague question.
Yes it will.
The longer version:
Yes it will but that doesn’t mean they will look the way you want. If you talk about UI, the scaling rules are decided by the UiCanvas settings and how you have done the anchoring etc., for the rest everything depends on the camera settings (orthographic/perspective, FOV/orthographicSize)
You can change the game view resolution from its top left corner so you can easily see how your game will look like on different screens.
When you get into the specifics of adjusting the camera, there’s plenty of Q’s and A’s about that here already so you should find help easily.