The default code in client.js attaches the mount point declared in index.js to the Default Surface in the React 360 app, where the Surface is a cylindrical layer for placing 2D UI. In order to use the 3D meter-based coordinate system for a layout in 3D space, we need to mount to a Location instead of a Surface. So update client.js to replace the renderToSurface with a renderToLocation.
/MERNVR/client.js:
r360.renderToLocation( r360.createRoot('MERNVR', { /* initial props */ }), r360.getDefaultLocation() )
You can also customize the initial background scene by updating the code r360.compositor.setBackground(r360.getAssetURL('360_world.jpg')) in client.js to use your desired image.
