I don't recall the specifics for 2020 but the attachments file is either a Disk Image file or (as in the latest version) a package file. For a disk image file you can rename it to end in .dmg and then double click on it to open it in the finder. For a package file (more recent versions have a .qbattach file extension in the name) control click on it and select "Show Package Contents".
Either way all the files are accessible from the Finder or whatever other program you want to use. They are stored inside of Folders with names that are uniquely generated (known as GUIDs techincally). The GUIDs match what they are attached to in QuickBooks (invoice, account, whatever) and are unique identifiers there, too.
I don't know of any program that preserves the attachment link when converting to QuickBooks Windows or QuickBooks Online.
