Considering its long history of supporting mods, it's only appropriate that Fallout 76 started life as a version of Fallout 4 modded with Quake multiplayer code. The project was a team effort of Zenimax Studios.
According to the lastest Noclip documentary on the development of Fallout 76, Bethesda Game Softworks asked what was then BattleCry Studios to try and retrofit multiplayer into Fallout 4 for a prototype. The team used Quake netcode that was given to them by id.
This all took place immediately after the release of Fallout 4.
There's a lot going on this week, to keep on top over everything we've put together a comprehensive E3 2018 guide.
Obviously, being built as a single-player game, Fallout 4 didn't lend itself to being converted into a multiplayer game, so it was a challenging conversion. As a nod to how the engine worked, the team called the main character Atlas, because he had the world on his shoulders - the engine and the game world all centred on him and his movements.
You can watch the full story in the Noclip documentary:
Now we know the Fallout 76 release date, Bethesda is being a lot more open what it has been working on.
Apparently, the prototyping continued to take place at the BattleCry studios in Austin, though it was absorbed into Bethesda's studios, being renamed Bethesda Game Studios Austin in March this year. Design of the game takes place in Maryland.