How does games screen strech/fitting work on mobile devices?

I wonder how does games screen streching/fitting work on mobile devices?

Is it based on the width and ignoring the height? So the width will always appear fully, regardless the height.

or is it based on the height?

Or is it both?

it works the same as pc. it depends on your scaling type. if scale outer it is both. if unbounded scrolling is selected it scales both left and right if not only to the right

see here

https://www.scirra.com/tutorials/73/sup ... reen-sizes

Ok, now it's more clear about the screen size support.

I've read the other screen size support tutorial.

So when they talk about the safe zone, does that mean they use: Scale outer mode?

And if I design the game on 1280x720 layout, will it be the same if I design the game on 768x432

since they are 16:9.

And does using smaller layout size, helps performance?

yes in the 16:9. and layout size should have no effect on performance