Debian’s documentation on installing the proprietary NVIDIA driver does not (yet) cover the "Bullseye" release because, as of the time of writing, it is still in testing. However, the documentation for Debian 10 "Buster" can still be used.
The main difference is that there is no "backports" for "Bullseye" – although I have found the folders on repo mirrors. At least to me they seemed to be there. Anyway.
What you have to do is to add "contrib" and "non-free" to your main mirror definition, not to backports. You don’t need to add a repo for backports because you’ll only get an error.
(Unfortunately, I haven’t made a note of the error message)
This is what you have to do:
Add "contrib" and "non-free" to the end of the main repo so it looks like this.
deb http://deb.debian.org/debian bullseye main contrib non-free
(The observant among you might have noticed that the feature image is from a Pop!OS installation, not Debian. I’ve since changed the distribution.)