I don’t know how the backpack works, could someone help me on how it works?

You can drag an asset/sprite/script to the backpack. Then you can drag it out to use it in another project.

You can find the backpack option at the bottom of the Scratch editor. Click on it and the space will open up. To move items into your backpack, just drag the required items like scripts, sounds, costumes, or sprites and drop them into your backpack.

Drag a sprite, script, costume etc. into the backpack, and any project you go, the backpack contains the items.

You can open the backpack, and drag some scripts into there and maybe some sprites, and then you are now backpacking, but make sure to credit the creator of that code

It's used to transfer sprites between projects. To add a sprite, costume, sound or script to the backpack, just drag it into the backpack area. It will auto-save. To transfer it to a different project, open the backpack and drop it into the project. To delete a backpack item, right-click it and choose Delete.