Setting up a Raspberry Pi board has always required a second computer, which is used to flash your operating system of choice to a SD card so your Pi can boot. But the Pi Foundation is working on a new version of its bootloader that could connect an OS-less Pi board directly to the Internet, allowing it to download and install the official Raspberry Pi OS to a blank SD card without requiring another computer.
To test the networked booting feature, you'll need to use the Pi Imager on a separate computer to copy an updater for the bootloader over to a SD card — Pi firmware updates are normally installed along with new OS updates rather than separately, but since this is still in testing, it requires extra steps.
It's something that many Windows PCs still can't do despite their modern feature-rich UEFI bootloaders (presumably in part because of licensing restrictions).
See
Raspberry Pi bootloader enables OS installs with no separate PC required#
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raspberrypi 
Beta bootloader is available now, will roll out to all Pi 4 boards later.
