If you are running Debian, it is strongly suggested to use a package manager like aptitude or synaptic to download and install packages, instead of doing so manually via this website.

You should be able to use any of the listed mirrors by adding a line to your /etc/apt/sources.list like this:

deb http://ftp.de.debian.org/debian bullseye-backports main 

Replacing ftp.de.debian.org/debian with the mirror in question.