When developing on mobile games, it is necessary to make sure the game is compatible on most of the screen size. Device Simulator is such a plugin or package which can easily show you:
- Playable area
- Game looks like under different model (iPhone/Android/Pad)
How to install device simulator
Window → Package Manager, click Advanced → Show preview packages:

In the search box search device simulator and in the popup window, select yes.

That is it, the installation finished. Now you can use the Device Simulator in the Unity Project: Window → General → Device Simulator:

You can click the Safe Area
to show the green border, the safe area is a feature that restricts the edges of game playable area so that they don't get clipped by the corners or notch (if there is any).
Reference:
https://docs.unity3d.com/Packages/[email protected]/manual/index.html