This is a tutorial on installing and burning the OS into the raspberryPi. Nearly any linux OS can be downloaded and used from the options are also given in the imager.
Raspeberry Pi OS is the officially supported opearating system for Raspberry Pi. It is available for free download on the linked website.
This process requires an SD card unto which the OS will be burned. The process involves formating, downloading of the image and burning it into the SD card. All of which are done by the imager .