Flutter, a recent cross-platfrom toolkit from Google, has the potential to be the dominant way of building mobile apps: one codebase gets you both Android and iOS apps. They have demonstrated using Flutter to build browser-side apps. And Fuschia, Google's possible new platform for Android and Chromebooks, uses Flutter. Finally, it's all open source so you could in theory build support for niche platforms such as those from Microsoft and others. Flutter features the Dart programming language, several libraries of UI widgets, hot reload that is somehow much faster than Android Studio's even on the same platform, and lots more. For now, see the Flutter Home Page.
Here are some resources: