Thank you for reminding me to add "bullseye-security". I went to Bullseye a while ago, because Buster didn't have my needed release level of qemu-kvm. But this is, pretty close to being a "production system".
Right now, those last two lines don't do anything, but they'll become
active once bullseye is stable. Note that the format of the security
line has changed slightly since buster, so simply replacing buster with
bullseye in a file from a current Debian release isn't correct.
You may also need to add bullseye-backports later, but you can worry
about those later, if you need them.
I only use Backports, if needed for a particular product.
If you intend to continue tracking "testing" even after bullseye
becomes stable, then simply use this:
deb http://deb.debian.org/debian/ testing main contrib non-free
There are no security updates for testing, so that's it.
Thanks!
Kenneth Parker