If someone says that they get to use your work without your permission because it’s in the public domain… they’re probably wrong. And just because something is on the internet definitely does NOT mean it’s in the public domain!
In copyright terms here in the U.S., “public domain” means stuff that isn’t protected by copyright, and anyone can use it without the permission of the author/creator/prior copyright owner.
Works falls into the public domain for a few different reasons. First, some things aren't subject to copyright because they aren't the stuff that's protected by copyright law in the first place: "creative works of authorship fixed in a tangible medium of expression". Those are things like facts or scientific formulas.
Second, some things are specifically excluded from copyright protection — like works made directly by the U.S. government. So feel free to use those NASA photos (not the meatball logo, but the photos) however you like.
The third category is works that used to be subject to copyright, but for which copyright protections have expired. And this is the trickiest category: because copyright law has changed, how old the work is matters. As of 2022, only works made before 1926 are usually in the public domain. Fun fact: the very first Winnie the Pooh book, created in 1926, JUST fell into the public domain this year.
And for your work: with very, very few exceptions, no one should be using it without your permission, and you really can get paid if they do. If you see that, come talk to a qualified attorney about what to do... and possibly how to get paid for the infringing use.