They Are Billions has always been a single-player game. The base-defense RTS, with its throwback design to the early Age of Empires series and its Blizzard contemporaries, has been about building walled cities that are safe against both roaming zombies who can infect your whole colony in a matter of minutes, and against the hordes that come sweeping in at semi-regular intervals and attempt to overwhelm your defenses. It gets harder and harder until you either survive a final swarm or you lose. Then you try again.
This is a companion discussion topic for the original entry at https://www.vice.com/en_us/article/qv7my5/they-are-billions-gets-the-single-player-campaign-nobody-needed