.sb3 structure - hayattgd/Emuratch GitHub Wiki
.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
.