If you are developing code for Bluetooth LE, a great place to start is the GitHub bluetooth-low-energy topic.
GitHub currently lists 877 public repositories that cover every operating system. This includes programming with Java, Javascript, C, Python, C++, Swift, Kotlin, C# and Objective-C.
In many cases it’s best to use the repositories to see how to do things rather than use the libraries themselves. Most, but not all, libraries are thin layers over operating system APIs. Most libraries don’t get updated and it’s otherwise easy to later get trapped into dependencies you don’t need, particular ways of working or old ways of using the underlying OS.
App cross platform frameworks are another source of problems when programming Bluetooth LE. They also aren’t updated often nor provide optimal Bluetooth APIs. If you wish to ease Bluetooth LE development and retain flexibility for future changes then use the native programming languages and libraries.