Newbie question: How to store custom data needed by my game?

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I'm an experienced dev, but I had no clue about any game dev. However, I was able to run the Atavism demo project MMO with my cloud own server in less than 5 hours. That just blew me away, honestly!

However, my dream game will need some more data entities (tables / structures) than what Atavism provides out of the box.

What options do I have to accomodate this requirement?

For instance, is it possible to:

• Extend the Atavism DB model by adding my own tables?
• Couple the Ativism realm with a firebase project (or any other 3rd party auth / DB service)?

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I guess I found the answer on how to extend Atavism if some feature doesn't come out of the box here:

• https://unity.wiki.atavismonline.com/project/creating-an-atavism-plugin/
• https://unity.wiki.atavismonline.com/project/agis-plugin-guild-example/