Hello, I would like to know if the projects should be totally original or if it is possible to use free assets and/or sound effects that can be purchased on the itch.io or on websites on the internet

Of course you can use pre-made assets! We know that with the limited time pre-made assets are really important, so of course we can't ban them/expect only assets made from scratch. Especially sounds etc.

So yeah, you are free to use them, just make sure to credit the people you took them from, just like usually :)