The Nvidia RTX 3080 series cards are not natively supported on Debian Buster yet. This was to be expected as Debian is usually quite conservative concerning up-to-date Kernel versions and newest drivers. Nevertheless cards are out since +6 months now.
My internet search did not bring up any usable tutorials that's why I take a few minutes to write up my half-way clean approach. (I do like to stick to debian vanilla as close as possible, using the drivers from nvidia directly is therefore not a feasible option for me).
1 - Enable Buster backports
The most recent drivers that support RTX 3080 series cards are only available in backports. https://wiki.debian.org/NvidiaGraphicsDrivers#Debian_10_.22Buster.22
Add this to your /etc/apt/sources.list or a file underneath /etc/apt/sources.list.d/
# buster-backports deb http://deb.debian.org/debian buster-backports main contrib non-free
sudo apt update
2 - nvidia-driver installation
Install the nvidia-drivers
sudo apt install -t buster-backports nvidia-driver firmware-misc-nonfree
Now this does not seem to do the trick as it pulls linux-headers-amd64 (5.10) but not the required (currently 5.10) kernel. After a reboot it still fails to load the nvidia driver and the desktop environment won't launch.
3 - Install Kernel (5.10)
As the required Kernel (currently 5.10) is missing. Install the missing kernel with
sudo apt install -t buster-backports linux-image-amd64
This pulls the required kernel from backports and builds the required modules for the nvidia-driver. I prefer to not explicitly install a specific kernel version but rather have it pulled from backports and also the subsequent updates that may come in.
4 - Reboot & enjoy working drivers
After a reboot your favorite Desktop environment, gnome here, should now launch.