All assets are getting downloaded client side in the user’s browser, sadly there aren’t many things you can do to protect your work. Eventually someone can go through the page resources and find ways to grab those assets.
That holds true even for native games, there are tools to grab the 3D geometry and textures directly from the GPU rendering them. So your best bet is adding a copyright notice to have a legal claim and stop and serious redistribution of part of your game.
Are you going to author a scene.json on your own? Usually we don’t really bother with it when using the editor, it gets exported automatically.
If you would like to take a look at it, just export a .zip build of a simple Playcanvas project and you can then open the scene.json (named as [sceneId].json) in your code editor.