When an Android device is plugged in to a PC (through USB?), is the internal file system mapped to a drive letter on the PC? So that one can copy files to and from the Android u
Yes, it CAN be mapped to a drive letter. It was much easier with something like a Droid X (Android 2.3), since it would show up as a mass storage device (disconnecting the card from your phone in the meantime), but it's also possible on newer phones such as the LG G3 if you're willing to install 2 free programs, which you probably would like to have anyway.
ES File Explorer - Remote Manager (FTP Server)
First, you need a file manager, not only because the built-in one is useless, but also because you need an FTP server. Install "ES File Explorer" on your Android Device. Then in the options, turn on the "Remote Manager" option, which will activate the FTP server and show you its local address like "192.168.1.3:3721. You can now access your phone as an FTP site from your computer when on your local WiFi network.
Net Drive - Remote Drive Mapping Utility
Next, all you have to do is install NetDrive: http://netdrive.net/ It's freeware and seems to be used by a lot of companies, because it lets you map cloud storage to local drives. That will allow you to map your FTP server on your phone as a local drive.
Don't bother trying to map an FTP site with explorer. First, you may run into a problem that's apparently caused by Chrome, where you can't even add an FTP network location. You'll get an unexpected error telling you the path format is invalid. Interestingly, that is solved by opening the registry editor, then under [HKEY_CLASSES_ROOT\ftp] add a key named "ShellFolder" with string value "{E436EBB6-524F-11CE-9F53-0020AF0BA770}". The error will immediately cease occurring, with no restart or any other action required. Just go back in and add the ftp network location. That, however, still does not allow you to map it to a drive letter, which is why you need NetDrive.