These libraries are provided with the hardware platform being used (usually just Arduino), and are typically essential to any development with the platform. Common examples are SPI, Wire, etc.

Arduino-CMake acknowledges that these libraries are essential, thus it provides a convenient way to use them in your application.
In order to use a platform library, one should simply link it to a target, usually the Arduino executable.

The following example shows how to use the SPI platform library:

link_platform_library(my_target SPI ${board_id})

Note that the example above assumes the my_target target has already been created earlier.
Also, board's ID has been retrieved as well.