Inside Bluetooth - Low Energy

This is the most important part for developers. It defines how data is exchanged using a hierarchy of Services and Characteristics (e.g., a "Battery Service" with a "Level" characteristic).

This is the hardware layer responsible for actually sending and receiving signals over the 2.4 GHz band. Inside bluetooth low energy

The Host acts as the software stack that manages how data is structured and secured. This is the most important part for developers