Website & app dimensions for UI

[GraviolaB17]

Hi guys!

Didn't find anything when I searched or perhaps I used the wrong search terms! But anyhow,
I've previously done UI elements in Designer and Photo for mobile app mockups that has been my ideas. I'm now gonna be the guy who creates the UI for app (iOS and 

For phones:
I understand that either the size I pick what's in the artboard is the screen and nothing else, so anything off the artboard will not show up and it'll be the actual screen sizes of the phones I can choose from.
When doing for iOS and Android and all different kind of models - are the sizes adaptable and what should I go for when doing the UI for a mobile? 2 or more versions for each size? Or will this be fixed inside the phone?

For web: 
Is there a fixed/normal size that will work for every page? and the programmer will just use this for the web design?
I am thinking that when I open Designer and go with a template, will this "work on any computer screen" or is there something I should be aware of before doing the design? Web has 17 presets I can choose from with different kind of page dimensions. So, what will happen with let's say top header bar with all navigation buttons and names etc?
Oh yes, and what is the preffed export files? PNG's for all icons and buttons or a .PSD or SVG? or something similar?

Any tutorial I have missed or any rules of thumbs here?
Any help is much appreciated!
I'm working on a 27" iMac retina.

Kindest regards!

[v_kyr]

For both you will probably find much more and better informations on the internet (via some topic related Google search) than here in the forum. - For website creation see also for example (Bootstrap, Normalize.css ... etc.).

[GraviolaB17]

Alright thanks v_kyr!