You can download it from the tools section in steam.
From the patch notes:
Now let see what this banana has to say...
From the patch notes:
Updated lobby and in-game UI to better show Additional Content details.
Added Sid Meier’s Civilization VI Development Tools, which includes ModBuddy, Tools, FireTuner, and the Steam Workshop Uploader.ModBuddy: A packaging and development tool.
Art/Asset Tools: Import FBX files into a format usable by the game, as well as customize existing art.
FireTuner: In-game debugging and editing tool.
Added Sid Meier’s Civiliation VI Development Assets, which includes the game art assets. This is a large download (approximately 7GB compressed download and 27GB on disk) which contains game assets, including models, textures, and interface elements.
STEAM WORKSHOPAccess mods created by the Steam community via Steam Workshop
Share your mods with the Steam community with the Steam Workshop uploader.
ModBuddy will also receive additional updates in the future, as part of a modding SDK update later. These tools do not include DLL source for Civilization VI at this time.
Now let see what this banana has to say...