See: https://onion.debian.org/
In particular, once you have the apt-transport-tor
package installed, the following entries should work in your sources list for a Debian system:
deb tor+http://vwakviie2ienjx6t.onion/debian buster main
deb tor+http://vwakviie2ienjx6t.onion/debian buster-updates main
deb tor+http://sgvtcaew4bxjd7ln.onion/debian-security buster/updates main
#deb tor+http://vwakviie2ienjx6t.onion/debian buster-backports main
InfectedMushroom wrote