I am sure everybody knows the progression D A B F♯ G D G A. Cord progression I V vi iii IV I IV V. Pachelbel's Canon in D, right? No. Just about any type of music steals those notes and that progression! That chord progression is super popular and called Pachelbel Progression, and you find it, or a variant of it like... everywhere. Even the Soviet Anthem is a variant of the Pachelbel Progression!
How can that be legal?!
First of all, copyright runs out. 70 years after the author's death, a work, including a chord progression, goes into the public domain. Afterward, everybody may use what was used in that work for free. No fees, no licenses, no negotiations.
Now, a second part is what is actually protected by copyright: each independent tangible expression is. So Pachelbel's Canon in D, if written today as a wholly new work, would be protected as sheet music.
But if you modify a work, you gain a copyright in what you add. So when the orchestra plays it, they gain a copyright on that recording and the sound engineer in the specific mix and setup used. Then a DJ Remixes the song and creates a different thing - and has copyright in how he mixed it with other music or altered the recording.
But two people taking the same sheet music can create separate recordings and both have copyright protection in their artistic expression. You could ask music fans and some will discuss the pros and cons of specific recordings, orchestras, and even how individual musicians influence the very same song.
So, there can be multiple different copyrights in basically the same song from the same basic sheet music. But what if we have different songs and they just pretty much sound the same?
There a different doctrine of copyright comes around: Scènes à faire. Scenes that you need to do. Certain Aspects of a work can't be protected. A rock band has a guitar, a bass, a drum kit, and a singer, Punk Rock has sick guitar solos, death metal is screamed into the microphone and a chamber quartet is 2 violins, a viola and a cello. Nobody has a copyright in a band setup. Even a rare setup like "4 Violoncello" isn't protectable - it's just another uncommon band setup. What can be protected is a particular arrangement for a song, and how this one sounds. So, if I take our loved/hated Pachelbel Canon and invert the whole thing, handing all the good parts to the cello and relegating all the underlying parts to the piano? The arrangement and recording are a tangible expression and protectable, but not the setup of "Solist and Piano", or even "Cello and Piano". And as we already know, the copyright in the original sheet music is gone.
Wait, I said the arrangement has a copyright? Yes, The arrangement is a variation of the original theme. It takes Pachelbel's notes and writes them out differently. In how it changes it or distributes it differently between different instruments, it makes specific expressions of the arranger. Those changes are copyrightable... unless they are, again, not creative. If I condense a song to a single cello and piano, there are only so many choices I can do. Anyone doing the arrangements has only a very limited choice between them.