The thing to remember is that there is ALWAYS copyright. The question is really about using music where the owner doesn't mind you using it for no charge. usuaing Google for copyright free brings back music that often has to be paid for, so no wonder everyone gets confused.
Songs can be a problem. Normally people want songs that people know, and if people know them, the chances are they will cost. It's very easy to mess it up. I recently recorded a George Gershwin track, and as he has been dead a long time, his music is in the public domain. However, the arrangement I used was NOT Geshwin's original, but a more modern one that is covered by the rights the arranger has. My solution was to base mine on the original, and avoid the intro, which was new, and a twiddly bit in the middle.
It's a very complicated subject and so easy to make mistakes!