That's a weird thing to say. These are all questions that copyright jurisprudence has had to deal with. The copyrightability of riffs should be well known to anyone who watched MTV in the Vanilla Ice years.
Why? Playing a melody over a different chord progression or a re-arragned progression, different instrument, add a few lifts etc. the possibilities are litterally endless.
You can't apply copyright to these things unless they are blatantly obvious, which they in most cases aren't and not something that really matters to the bigger picture.
But the actual act of copying them. Most music is composed that way.
Thats also called inspiration. Which is just a nicer way of saying a less obvious way of copying.
