How can I determine which download to use?
There is no simple answer to this. A device may have a 32bit processor, or a 64bit processor in combination with 32bit -or- 64bit (android) operating system.
Only devices with a 64bit processor and 64bit operating system will require the Android64 software build.

Try the Download for Android 32bit devices first, and if it misbehaves then uninstall it (if it gets that far) and try the Download for Android 64bit devices.
The Download for older Android 32bit devices is mainly for those with Android 5.1.1 but may run on some newer 32bit devices.