The easiest way is to use the product properties where you can add arbitrary information without extending the database or the managers/items. You can even provide a new subpart and template with specialized fields like the "Physical" subpanel to improve usability (weight, height, length and width are stored as product properties).
If you want the country of origin to be an option in the facetted search, you have to use attributes you associate to the products instead. Properties doesn't generate facetts.
For extending the admin interface there's no documentation yet. You have to have a look into the existing code to see how it works and copy/paste the things you need.
If you want to extend the database instead, you should read these articles:
Add tables and fields to the database: https://aimeos.org/docs/Developers#Database
Create or extend managers/items: https://aimeos.org/docs/Developers#Managers_and_items