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Most, if not all, write to the blockchain as custom JSONS. The application can then read these custom JSONS to track what they want to track (e.g. Splinterlands card sale, etc)

thanks, that clarified things!

Here's a quick summary: user actions are saved onchain but the computations associated with those actions are computed offchain (i.e. not by hived nodes but by 2nd layer app code). Since all the data necessary to reconstruct the state of the 2nd layer app is stored onchain, the 2nd layer ecosystem is decentralized as long as the app is open-sourced because you could run the 2nd layer app code on your system and get the same results as anyone else would, hence there is no centralized server required.

thanks that helped :)