Control any system/OS. This Teensy project utilizes a blue-tooth module for wireless connection to any device. For engineers who are required to attend with their own usb keyboard it is an excellent little tool. There is an app available on playstore which provides keyboard, US/UK layout, and track-pad functions but you could develop your own and simply send the commands used in the sketch.
! This device would also make an excellent macro player which would work with any OS or HID compliant device. Your app or any micro system can send keyboard commands without having to install anything on the client system. For instance, visitors to a venue or establishment could use an app to control systems there with dedicated buttons that performed specific tasks, thus limiting that users functionality to the app they have been provided with and providing a simple OS independent programming capability. :-)
Full build instructions and the sketch are freely available here- https://www.instructables.com/id/Engineers-Buddy-Bluetooth-Kbd-and-Mouse/
All feedback will be very welcome