The "bébé keycube" (BBK3) is a controller-like device that can be used for multiple usages. it is a smaller version of the keycube so it possess less keys but it is easier to handle. In this project it can be used as a gamepad to play some games. The chip inside the BBK3 is a XIAO nRF52840 Sense. It uses the chip's motion sensors as joystick inputs.
There shouldn't be anything you need to download to use the BBK3. If you want to edit the code, you'll need either the arduino IDE if you edit the C++ version or python with a python IDE (I recommend MU editor) if you edit the python version.
You can use the gamepad in games. The BBK3 has mostly been tested through unity. The C++ version needed a custom layout to work properly so it may need some kind of accomodation in other game engines too.
You can use the gyroscope's and accelerometer's inputs to control an object's position and rotation, for example, in the unity project in this repository, you can see a cube. It's rotation is controlled by the gyroscope's inputs.