Function mount_scene_content 

pub fn mount_scene_content(
    render: &WorldHandle,
    pass: RenderEntity,
    panels: Option<PanelSlots>,
)

Queue this scene’s own content on the render world: its camera, and — where there is a panel to draw — the UI source that draws it.

pass is the pass returned by setup_render_world, where the panel’s source belongs.

Both are jobs, so a world that was set up to draw can be given a scene without its setup ever hearing of one. The panel itself lives in the gameplay world and nothing here reaches it: the source draws whatever the slots hold, and the world holding the panel fills them.