Function extract_ui_layers 

pub fn extract_ui_layers(
    commands: Commands<'_, '_>,
    phases: ResMut<'_, ViewSortedRenderPhases<TransparentUi>>,
    extracted: ResMut<'_, ExtractedUiLayers>,
    layers: Extract<'_, '_, Query<'_, '_, (Entity, &LayerCaptureRect, &LayerGroupAlpha, &ComputedUiTargetCamera, Option<&ResolvedFilterChain>, Option<&ResolvedBackdropChain>, Option<&LayerTransform3dMatrix>, &PromotedLayer, Option<&ComputedNode>)>>,
    membership: Extract<'_, '_, Res<'_, LayerMembership>>,
    repaints: Extract<'_, '_, Res<'_, LayerRepaintState>>,
    clips: Extract<'_, '_, Res<'_, LayerClips>>,
    cameras: Extract<'_, '_, Query<'_, '_, (RenderEntity, &Camera), Or<(With<Camera2d>, With<Camera3d>)>>>,
    main_pass_formats: Res<'_, CameraMainPassTextureFormats>,
    store: Res<'_, LayerTextureStore>,
)

Extracts promoted layers into the render world and spawns their synthetic capture views. Must run after extract_ui_camera_view: that system ends with a retain that would drop any phase it didn’t create.