JUCE is a great platform for building multi-platform applications with C++. You can build iOS, Android, macOS, Windows and Linux applications from the same codebase. JUCE handles most of the platform specific configurations so you can focus on writing C++ code.
It is particularly great for developing audio applications but you can just skip the audio stuff if you want to and do something else. Developing a good UI with pure C++ will be difficult if not impossible so you probably should learn a multi-platform UI framework like Flutter or React Native if you want people to enjoy using your applications. Swift is great if you want to focus on just iOS.