Two Apple IDs, but which one bought what?

Cursed historical nuisance but I have two Apple IDs. How can I see which apps, films, etc. were bought on which Apple ID? It seems to me that sometimes macOS throws me one or the other to sign in for a new purchase, but which is consistent for that device? Same for iOS. Oh, I so wish I could merge them but that has never been possible for reasons best known to Apple