The Juego de la Rana app is created in iOS-native Swift using the RealityKit framework, which uses a "Universal Scene Description" (.usdz) file format for importing 3D model files. This is Apple's ...
b) is not that useful if you desire a bigger map with more variation and c) takes just really long. For a) there are a lot of terrain generators like world machine, world creator, gaia, gaea, etc. But ...