Reports of Failures with macOS/Mac OS X and Android File Transfer.app
A quick search on DuckDuckGo or Google when you encounter the "No Android device found" dialog from the 2012 (latest) version of Google's Android File Transfer.app will tell you about multiple others seeing the same problem. Too few (none?) have the following solution.
Launch Order Matters
Do the following in order to have Android File Transfer.app show your Android phone or tablet's file contents.
- Connect your Android phone or tablet to your Mac (via USB-C, USB 2/micro-USB, or an adapter between the two, as necessary)
- Leave your device in "USB charging this device" mode
- Launch your Android File Transfer.app, and wait for "No Android device found"
- Switch your device to "Transfer files" mode
- You now have Android File Transfer.app displaying your device type along the top of its window, and your folders in the main pane of the window
- Transfer to your hearts content