Check out example codes for "insert nested documents using Mongoose". It will help you in understanding the concepts better.

Code Example 1

Post.findOneAndUpdate(
    {"_id": req.params.id}, 
    {$push: {comments: {
        comment: "Hello World",
        user: "933ujrfn393r"
    }}
}).then(function (post) {
    console.log(post);
    res.json({success: true});
});

Learn ReactJs, React Native from akashmittal.com