The actual generation is currently done via taskcluster cron. The state that is good to use by localizers at large is published at https://hg.mozilla.org/l10n/gecko-strings/.
The L10n team is doing a :ref:`review step <exposure-in-gecko-strings>` before publishing the strings, and while that is ongoing, the intermediate state is published to https://hg.mozilla.org/l10n/gecko-strings-quarantine/.
The code is in https://hg.mozilla.org/mozilla-central/file/tip/python/l10n/mozxchannel/, supported as a mach subcommand in https://hg.mozilla.org/mozilla-central/file/tip/tools/compare-locales/mach_commands.py, as a taskcluster kind in https://hg.mozilla.org/mozilla-central/file/tip/taskcluster/kinds/l10n-cross-channel, and scheduled in cron in https://hg.mozilla.org/mozilla-central/file/tip/.cron.yml.