they r royalty free 

I believe that the idea is to create all your own assets regardless of if they are free or not

ok thanks 

Clarification, you can use stuff like TMPro and other smaller but very good stuff like https://assetstore.unity.com/packages/tools/particles-effects/quick-outline-1154..., but you should put what you use in the description of your game. I will be lenient on that sort of stuff.