I’m talking about an individual, not a business scenario. Your papers in your custody in your home are only accessible via warrant, as opposed to your online stuff that requires as little as an administrative subpoena.
Your doctor and attorney are unique in that professional relationship is legally privileged.
I'm just saying the fact that everything in the legal business is classified by "custodian", without any more context, kind of indicates that the default is not to assume ownership in the manner you're suggesting. Like, when something is legally disputed, there's a good chance people are hiding things, giving them to others, intermingling evidence with other things, etc. The system couldn't work if it assumed that everybody followed the rules. No, I'm not a lawyer, but come on.
Your doctor and attorney are unique in that professional relationship is legally privileged.