Does it diff the JSON string, or the objects/array themselves? Does it serialize the data or leave as plain text?
I ask, because I have a similar setup, that needs to sync dynamic data structures, but also keep the data deltas as small as possible when sharing them as JSON strings to save bandwidth.
It will not automatically diff these with the previous value (in a distributed system it's difficult to say what the "previous value" was, but it will sync only those properties that have been changed). For text heavy operations, you can use the Text structure which will sync fine-grained operations ([delete 4 characters at position 2] or [insert "world" at position 5]). This makes it suitable for collaborating on rich text documents (similar to Google Docs), see: https://syncedstore.org/docs/advanced/richtext.
This might also be an interesting resource to learn how the Yjs internals work (SyncedStore builds on top of this): https://github.com/yjs/yjs/blob/main/INTERNALS.md