When you install a new version of Debian from scratch, you may need “non-free” drivers for things like a Wireless Network Adapter. If these drivers are not GPL-licensed, then Debian will not include them in the installation image.
You can work around this by installing the non-free image from a location such as http://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/10.7.0+nonfree/amd64/iso-cd/ for the current version of Debian Buster.
However, even the non-free image might not have all the drivers you need especially if it is a “netinst” image:
The netinst CD here is a small CD image that contains just the core Debian installer code and a small core set of text-mode programs (known as “standard” in Debian). To install a desktop or other common software, you’ll also need either an Internet connection or some other Debian CD/DVD images.
During the Debian installation, you may be prompted to provide a USB drive with the required drivers. There is no explanation of how to do this. If you search for the required driver, you can find out where to download it. To make it available to the installer:
- Download non-free firmware .deb files from debian repos like
firmware-brcm80211andfirmware-realtek - If you are installing from a boot-able USB drive, you can remove it and add a subdirectory named “firmware”
- Copy the firmware .deb files to that subdirectory – no need to extract them from the .deb
- Re-insert the USB drive and tell the installer to search for the required files.
You could also go to http://cdimage.debian.org/cdimage/unofficial/non-free/firmware/ and download all the non-free .deb files for your Debian version. Extract the .deb files from the archive and place them in the firmware subdirectory.