You keep making similar statements about the "Public Domain" as if it means anything posted in "public" is free of copyright... That's not how the "Public Domain" works or what it means...
All works (content), regardless of whether it is or isn't posted in private AND public, by the very nature of simply existing IS copyrighted. Yes, the cute picture you took of your dog/cat last night is copyrighted and you (the person who clicked the button) holds that copyright. You don't have to register it or do anything, you automatically hold the rights to that photo. (I'm not going into works for hire or works created in your normal job function at your employer)
This is a simplified explanation:
Think of the "Public Domain" as an imaginary copyright holder of works that were created before 1927, some works created between 1927-1978, works that have had their copyrights expired, surrendered, forfeited, or explicitly waived, and works created by a public entity (Court documents, congressional documents, things created by the government like NASA photos, USGS pics/data, etc.). This entity cannot exercise any rights as a copyright holder because they are imaginary and they're just a cool imaginary being.
Posting a work in public does not waive any rights of the copyright holder AND does not grant you any license to copy, display, or distribute said works.
Yes, I've used examples of entities who's works fall into the public domain that are US centric, the concepts are still the same as most countries have adopted almost identical copyright laws.