Hi there,
I’m new to the Unreal Engine. I’m using Unity 3D right now and I’m looking over to Unreal 
How does Unreal handle multiple screen-resolutions? Does it automatically scale the UI proportionally like Unity?
Unreal doesn’t have a pre-built resolution selector the way that Unity does, but it has very robust support for different resolutions and quality settings. You can specify default values in your GameUserSettings config file. Most games try to make a “best guess” at resolution and quality settings and then provide a UI in their menu systems to change the resolution and other quality options. There are many AnswerHub posts that show how to handle determining resolutions and setting resolutions and other related values like vsync, AA, etc.
Here’s one of them.
UMG, Unreal’s UI toolset has a number of tools for dealing with variance in screen size and aspect ratio. I’d suggest following one of the UMG video tutorials if you want to know more about how to handle it, but generally speaking, yes, it will scale the UI, but it will do it how you tell it to. You can specify whether widgets scale or move in the editor using anchors.
tl;dr Unreal has great tools for handling multiple resolutions and quality settings, but they’re not quite as pre-packaged as Unity’s resolution selection dialog.