Hi Cheik88,
Assets are the parts that make up the characters.
You can add new parts in unity, however the standalone version doesn't support this.
With purchase however, you do get bother versions of the program (If you buy through itch)
You can see how to add your own assets by looking at THIS: https://docs.google.com/document/d/1-2myJe7zy6jmImRGheBWq7l_bqAtTnhTYiy_T9RRukM/edit#heading=h.al0it5jsgp54 link.
Cheers,
Kombowz