What determines game or app differences of availability between countries?
Two factors

Google for reasons not made public has a list of countries where  app and Google Content is available Country availability for Google Play apps &amp; digital content.

Developers may choose targeted release for their own reasons. Google guidance on this - Distribute app releases to specific countries - Play Console Help. I am not a developer but I guess reasons could include localization, scalability of app, revenue expectations, taxation issues, political reasons (Indian ban on Chinese apps recently comes to mind), reviews (I am aware of a famous developer who doesn't release his apps in some countries because of unfair reviews).