The answer is nobody. If you have 10,000 albums, CD’s or physical books, they pass on to your heirs through your estate. If you have 10,000 tunes in iTunes or 10,000 books on a Kindle, your heirs get nothing. You don’t own those files, you license them, and the license is non-transferable.
Of course, if your digital files are unencumbered by DRM, as a practical matter you can do what you want with them. The license may say they are not transferable, but there is nothing Apple or Amazon will be able to do about it. However, if your entire music collection is in the Apple cloud, Apple can make it all go bye-bye.
Probably a more pressing problem than the inheritance issue is the company or product survivability issue. If Amazon goes bankrupt in 10 years all your Kindle purchases will become unavailable to you when the DRM servers are shut down.
Music files from Apple or Amazon are not encrypted by any kind of DRM. However, I believe they are tagged with unique identifiers. Kindle books are most definitely encrypted. That is the primary reason I won’t buy anything for the Kindle that costs more than about $6.
As more and more of our lives move online, these issues are going to continue to mount. Now who wants to die we can establish precedence in court with your Kindle files?