The “versionCode” is for anything android related (not just Play Store). The “version” value is a human-readable string, which is for you and potentially for users to understand what version is currently installed. “versionCode” is simply an integer to help the operating system track versioning of the build (as it can’t understand the “version” string).
If you simply increment the “versionCode” every time you make a release of your game, users downloading that build will get the option to upgrade the app, which will keep all saved data of your app, so users won’t have to start over with each release.