Hello all 
I was wondering if there is a way to seach within the Epic Games Marketplace for specific “free” assets? I’m new and I would like to download some for testing and research. Also, once downloaded the files where do they go? How to access them? Thank you so much!
Simply start the launcher 
There you will find a ton of free content provided by Epic as Learning Ressources (left hand side, the button “Learn”). You can access everything from there.
For most items on the Marketplace needs to be paid for. As order66 said, the Learn tab in the UE4 Launcher is full of free stuff. As far as I know all the assets from the Learn tab can be used in your own project, even commercially without payment.
A few interesting free items on the Marketplace:
https://www.unrealengine.com/marketplace/animation-starter-pack