"Install an OS onto boot media

To use your Raspberry Pi, you must first install (flash) an operating system (OS) onto your storage device, making it a boot device. By default, Raspberry Pi devices check for an OS on any SD card inserted into the SD card slot. Some Raspberry Pi models allow you to boot an OS from other storage devices, such as USB drives, storage connected through a HAT, and network storage. The following instructions assume that you're using a microSD card.

We recommend installing an OS to your storage device using Raspberry Pi Imager on another computer. For instructions, see ~Install using Imager. If you don't have access to another computer for writing an OS image to the storage device, you might be able to install an operating system directly on your Raspberry Pi from the internet. For instructions, see ~Install over the network."

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What is the difference between a "recommendation" and a "default"

Here, we can see that this is a recommendation not a default

The owner must make a choice

The company has not pre-selected a default choice

It has only provided recommendations

Contrast "defaults" with "recommendations"

The former is opt-out; a choice has already been pre-selected by the company

The later is opt-in; no choice has been pre-selected, a choice must be made by the owner