Reference Layer feature was apparently added to the latest iOS app update. However, there is no mention of how to use reference layers in any of the guides or tutorials. Nor is there an obvious icon designating a layer as a reference letter. How to use the new reference layer future? Credit: appstore update notes
You can designate a layer a Reference layer by choosing that option in the layer menu. Whatever you draw on that layer can be filled with solid color on another layer. Once you choose the reference layer option, an icon appears on the layer.
It works with the fill bucket, so create a new layer, then tap an area with the fill bucket and the color will fill the layer within the area of whatever you drew on the reference layer.
I think it's so new, we're not seeing more info online, but it is inside the app. You may need to reset your learn content to see the pop up in the app. Go to App Settings > General > Restore Learn conent.