Epic Games has an enormous amount of FREE material libraries that rival in aesthetics the industries best rendering software including what you've seen in KeyShot, V-Ray, Arnold, Octane, and Redshift.

A large majority of Unreal Engine materials are based on PBR (physically based rendering) techniques that recreate the material properties to be as close to the true-to-life material appearance.

So!
This is your Go-To free material collection:
https://www.unrealengine.com/marketplace/en-US/slug/automotive-material-pack

Here's a McLaren 570S demo using the free automotive material collection
https://www.youtube.com/watch?v=l4FiJ1A5veY

Here's a Porche 911 demo using the free automotive material collection
https://www.youtube.com/watch?v=QmbvSkBBrmI

Among many other FREE plugins for materials, here's another great addition for your Unreal Engine projects
https://www.unrealengine.com/marketplace/en-US/slug/gametextures-com-2019-free-material-pack

All and every material used in the Unreal Engine can be completely overhauled, edited, and made dynamic to suit any project. You just can't go wrong!