The native file format for Scratch 3.0 projects. It is essentially a renamed ZIP archive containing a project.json file and various media assets (sounds and costumes).
Even after converting your Zip to SB3, you might encounter issues. Here is how to fix them: Zip To Sb3 Converter
: While primarily for converting Scratch to HTML, it is a staple in the community for managing Scratch project formats. PenguinMod Packager The native file format for Scratch 3