Hello Toby,
Happy to answer your questions 
- We used Xcode (Apple’s IDE) and the Swift programming language to build the mobile app, and are using Android Studio and the Kotlin programming language to build the Android version. Our server code is written in Java Spring with a Mongo DB database and deployed on AWS (Amazon’s cloud service). Depending on the complexity of your UI, you may want to consider REACT Native or something similar for the mobile app, as you can potentially save dual work in building apps in two separate languages. Swift and Kotlin are very similar and we use some libraries that work on both, so the cross conversion is not too bad, and you get more control by having a purely native app on both platforms.
- There are several alternatives to DarkSky. The cheapest is Open Weather Maps, however I find their forecast data to be just “OK” in the US. I think it’s better in Europe. They are based out of the UK. We also integrate with visualcrossing, weatherstack and NOAA. Each are unique integrations, so you just have to put in the work. In our case, the user can select/change weather sources as they need. We also use several Google APIs and get the KP Index for Aurora from NOAA.
- All of these services provide REST styled API integrations. You integrate with them using standard HTTP messages using a JSON payload.
- Experiment, experiment, experiment. Don’t get attached to your original design, get feedback and have fun.
If you want to check out our app, you can find the link to the app store here: http://light-watch.com
Good luck with your project!
Wayne