To archive mail, add email@example.com as a delivery target. This will cause the archive machine (currently finch, the MX record for archives.gentoo.org) to receive these messages.
To configure the webUI: do more things! I suspect you have to edit some mailing list lists in puppet and do some other stuff in the mailinglists.yml of api.gentoo.org someplace.
MX / mailspool
The MX machine for archives.gentoo.org is finch. Mail is delivered there to the garchives user. The garchives user has a homedir in /var/www/archives.gentoo.org and receives mail there; the messages are delivered to procmail where they are sorted into per-list maildirs by procmail and also given dated subfolders (e.g. .gentoo-dev/.202001/new would be messages received in that month.
Manakin is "archives.gentoo.org" and runs both the backend (elasticsearch) and frontend (serving app) for archives.gentoo.org. A cronjob running as garchives runs every 10m on manakin. This job syncs the maildirs from the MX host (finch) to manakin. It then scans the maildirs for any targets that have "recent" messages. Recent is currently configured as 6 hours. These lists are then fed into an indexer that indexes the messages and makes them available on the archives.gentoo.org web frontend.