I has shared a sample cordova plugin project in response to your previous question related to queries. You should follow the same pattern as specified in the example for the other APIs .
Implement the document insertion logic natively in the plugin and expose a JS API that takes in the JSON. All of our native APIs are strongly typed so you will have to create MutableDocument type.You cannot pass in raw JSON string.
If you are looking for examples of how to do document creation natively , you can look at one of these other sample projects…there is a React Native module one as well.