What determines game or app differences of availability between countries? There is a particular game which is in beta release in on country, in my country it is possible to register on a waiting list and in South America it just says &quot;not available in your country&quot;. Is there a general explanation why the availability is different between different countries? Is it for marketing and purchase power adjustments or some other reason?
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).