>You cannot send someone a LUKS volume to decrypt one file, and backups of a mounted encrypted volume are plaintext unless you add another layer.
Veracrypt, and I'm sure others, allow you to do exactly this. You can create a disk image that lives in a file (like a .iso or .img) and mount/unmount it, share it, etc.
You can still do that with a .dmg, for example. I've done it, it works more or less like a zip.
But even if that was somehow unreasonable or undesired, you can use Filippo's age for that. PGP has no use case that isn't done better by some other tool, with the possible exception of "cosplay as a leet haxor"
Veracrypt, and I'm sure others, allow you to do exactly this. You can create a disk image that lives in a file (like a .iso or .img) and mount/unmount it, share it, etc.