None of it is "legally" mandated, i.e. there are no laws specifying what a recording artist should get paid. All of it is a complex series of contracts and righ

None of it is "legally" mandated, i.e. there are no laws specifying what a recording artist should get paid. All of it is a complex series of contracts and rights agreements between various corporate entities that have generally become standardized, in order to eliminate negotiations for every song or every artist. But that doesn't mean these agreements are necessarily fair or equitable to all parties.