Tonydax,
Something is in the public domain if nobody currently owns the rights to it.
You can do a new comic using the characters, so you own that comic but not the rights to the characters.
CB+ doesn't make money out of any of the books posted here.
Others do, and publish books of the Golden Age books we have here.
I don't know exactly how it works with the Spirit. Someone will still own the copyright, so you can't make new stories with the character. But the original works were in Newspapers and in Quality comics and nobody owns them, so we can post them. I think.
To learn more, go up to the search bar on CB+ and search for 'Public Domain'. It will take you to many threads on the subject.
cheers!