The Fortix giveaway has me curious about how massive scale giveaways operate

Featured just means it's about to expire. The developer giveaways on the other hand submit the keys to the site in advance, that then randomly gives them away to people who enter. It is automatically marked received, because, as you saw, the keys are distributed through the site on your received games page.

Yeah, they give the keys to CG and he sets it up to work that way.

If a user wants to hold a featured giveaway (not the front-page-now-ending kind), or if a developer wants to hold a developer giveaway, all they need to do is to contact support or the site administrator. The number of copies and the keys that'll be given are handed to the site administrator, you create a giveaway with the time settings you want, and then that giveaway is edited to be "Featured"/"Developer" along with the number of copies you intend to give away, and is placed on top of the giveaway list. After the giveaway ends, all keys appear in the winners' "View Won" page, and the giveaway is automatically marked as Received.

If you are a dev/user that want to give a major amount of copies (>50), you can have your own dev/feat giveaway too.

Devs upload the keys together and the system sorts out the keys and give to individual winners.

Devs just post numbers and letters randomly in each account and they hope that some Fortix come out of that :D