i always wanted to make a platformer like @griffpatch 's scratchnapped how do you make the backround look like a level?

Paint it… In the paint editor. Seletct stage in project editor, go to backdrops and then you can paint your background for your game.

Alternatively, you can import a preset image as a background by clicking on a painting icon under where it says “New backdrop” and you can select a background from Scratch's background library.