Question Which DPI (resolution) of Photoshop to choose

Hi! We've been developing a 2D project for over a year and we all use Photoshop. And today my artist asked the question "why do we have different resolutions everywhere?"
Until now, I thought "resolution" was just some kind of height to width ratio.
As a result, we found out that "resolution" is dpi, affecting only the aspect ratio in Photoshop and the printing (as i understand).



Based on this, I have 3 questions
1) Does "resolution" really have no effect on files within Unity? We experimented - sprites look the same with different "resolution"
2) What "resolution" do most people use? 72? 100? 300? As far as we understand - it's better to have all works in the same "resolution", because png in different "resolutions" are rescaled relative to each other.
3) Question on another topic - is it normal if in our game locations(scenes) have different size in pixels? For example, the room of the main character is 1920 * 1080. The street 3840 * 2000. Is it okay?

Thank you! That really made me really worried.

 

The pixels/inch does not matter. It is only useful if printing out your graphics for some reason.

You can have each image a different size, but it is often best to keep to power-of-two sizes (256, 512, 1024, etc.). It can be more memory efficient in many circumstances (I don't know the latest Unity methods for handling it, but it used to be important), and is needed for many of the compression options.