Each instance has complete control over what is posted on it. The only way to truly own your posts is to set up your own instance and interact only with communities on it.
An interesting thought. I’m not sure this is entirely true though in many jurisdictions. It is clearly possible to post something on someone else’s server and still maintain ownership of it. Platforms like SoundCloud have you specify a license in the ui client at the time of upload. While this might seem performative, it is explicit.