/etc/apt

See also

$ apt-config dump | grep 'Dir::Etc '
Dir::Etc "etc/apt";

$ apt-config dump | grep  Dir::Etc:: | column -t
Dir::Etc::sourcelist        "sources.list";
Dir::Etc::sourceparts       "sources.list.d";
Dir::Etc::main              "apt.conf";
Dir::Etc::netrc             "auth.conf";
Dir::Etc::netrcparts        "auth.conf.d";
Dir::Etc::parts             "apt.conf.d";
Dir::Etc::preferences       "preferences";
Dir::Etc::preferencesparts  "preferences.d";
Dir::Etc::trusted           "trusted.gpg";
Dir::Etc::trustedparts      "trusted.gpg.d";
Dir::Etc::apt-file-main     "apt-file.conf";