But these days purchases show up on both devices whether they’re downloaded or not. There’s a little cloud or arrow symbol beside them if I want to download them (eg Apple Music). So it’s confusing that there’s a separate policy that insist on both devices having the same content.
But I see in macOS Finder, when I navigate to my device there’s a checkbox enabled labelled “Sync music onto iPhone”. So maybe this is why (and I likely set that myself, I rarely ever connect my phone these days).