Go to where your Civ6 folder, on my computer it's : Documents\My Games\Sid Meier's Civilization VI
look for "AppOptions.txt"
open it with a text editor, Notepad will do
search for the text " :EnableDebugMenu 0"
change to "EnableDebugMenu 1"
save the file
now start a game, and look for the tilde key on your computer " Function ` " (it's usually on the left next to the 1 key), hit that key to open Debug menu
in the text window that opens at the top of the screen you can use several commands
type "Reveal all" will meet every civ and city state, so be prepared for a bunch of notifications
note: "explore all" will do something similar, but I believe restore the fog of war