.sb3 file is basically .zip file, meaning you can extract it by changing its name.

Inside .sb3 file, there are mostly 2 types of files.

project.json

This file contains data of sprite, blocks. Basically, everything is contained here.

Reference wiki page for more information.

Assets

.png, .svg and .wav for costume image and sounds. Images don't contain margin and offset are saved on project.json.