Layers are on our roadmap, but aren't an immediate priority. And data sources are not on our roadmap. If these become higher priority for users, that could change in the future. You are correct in that the web has more advanced options than the app and it is a view-only experience on the mobile browser. If you would like to email us at firstname.lastname@example.org we'd love to understand your use case for layers and data sources on iOS. We value user feedback!