How do I make a DataStore that works between different games?

As far as I know, you can only do this through game universes, unless you make a website and use HttpService between games to record and load that information. If you are just using game universes, the data should automatically translate between universes.

Datastores can only be accessed in other games if the game is set up in a universe.

More information can be found on this wiki page: http://wiki.roblox.com/index.php?title=Game&redirect=no#Managing_Places_in_a_Game

Once you set up another place inside the main place you can get access to the global datastore by:

local dataStore = game:GetService("DataStoreService"):GetGlobalDataStore();

or get a specific dastore:

local dataStore = game:GetService("DataStoreService"):GetDataStore("Coins");

In both cases, you should be able to access the datstore and get the information iniside. (not tested, and haven't worked with datastore in awhile, so hopefully this is correct, lol)

edit: you and also do it through HTTP requests (google spreadsheets, firebase, trello, etc...) like mattscy said above ^