Accessing/scanning the iOS Downloads folder (the one airdropped-to) from an app

Here's the problem I'm trying to solve: Create an iOS app which can scan the Downloads folder (where airdropped audio files arrive), identify audio media files, and play them, retaining some of its own metadata about them (basically, create textual notes mapped to timestamps and store that information in the apps own storage).

I am not able to access that folder. I am able to get a path from

NSSearchPathForDirectoriesInDomains(FileManager.SearchPathDirectory.downloadsDirectory, FileManager.SearchPathDomainMask(arrayLiteral: FileManager.SearchPathDomainMask.userDomainMask), true)

or a URL from

NSSearchPathForDirectoriesInDomains(FileManager.SearchPathDirectory.downloadsDirectory, FileManager.SearchPathDomainMask(arrayLiteral: FileManager.SearchPathDomainMask.userDomainMask), true)

but

let fileUrls = try fileManager.contentsOfDirectory(at:downloads, includingPropertiesForKeys: [])

fails with an error that the folder does not actually exist, with or without a call to downloadsUrl.startAccessingSecurityScopedResource().

Determining whether this is a permissions issue, or if I'm getting a URL to an application-container local folder that has nothing to do with the one I am looking for is compounded by the fact that if I set the build setting Enable App Sandbox, then deployment to my phone fails with Failed to verify code signature. I have spent hours trying every possible combination of certificates and deployment profiles, and ensured that every possibly relevant certificate is trusted on my phone.

Disable app-sandbox and it deploys fine, either with automatic signing or an explicit cert and profile.

I have an entitlements file with the following - though, without the ability to enable app sandbox and run it on a phone with actual contents in the downloads folder, it is probably not affecting anything:

	<key>com.apple.security.files.downloads.read-only</key>
	<true/>
	<key>com.apple.security.files.user-selected.read-only</key>
	<true/>
	<key>com.apple.security.app-sandbox</key>
	<true/>

So, questions:

• Should the URL returned by the above call be the Downloads/ folder airdropped to in the first place? Or is it a URL to some app-local folder that does not exist?
• Does the entitlement com.apple.security.files.downloads.read-only even allow an app to list all files in the downloads directory (presumably asking the user's permission the first time), or does the permission only get requested when using a picker dialog? (the point here is to find any new audio files without making the user jump through hoops)
• If I could get it deployed with app-sandbox enabled, would the above code work?

Backstory: I'm a software engineer, audio plugin author, Logic Pro user and musician. My workflow (and probably many other Logic user's) for work-in-progress music is to airdrop a mix to my phone, listen to it in a variety of places, make notes about what to change, edit - rinse and repeat. For years I used VLC for iOS to keep and play these in-progress mixes - you could airdrop and select VLC as the destination (yes, Logic can add to your Apple Music library, but trust me, you do not want 20 revisions of the same song cluttering your music library and sync'd to all your devices).

Last year, the behavior of Airdrop changed so that the target app for audio is always Files, period, wrecking that workflow. While I eventually discovered that, with an elaborate and non-obvious dance of steps, it is possible to copy files into VLC's folders, and make them available that way, it is inconvenient, to say the least - and VLC is less than fabulous anyway - it would be nice to have an app that could associate to-do notes with specific timestamps in a tune, A/B compare sections between old and new versions and things like that.

So, figuring sooner or later I was going to get into a car accident futzing with the Files app to listen to mixes while driving, perhaps I should write that app.

But the ability to do that at all relies on the ability of an app to list and access the Downloads folder airdropped audio files land in (assuming the user has given permission to access it, but that should be needed once).