aptX is a compression standard for audio that maintains the full range of audio frequencies while still compressing the music into a bandwidth that bluetooth devices can handle. This ensures better sound quality, even over Bluetooth.
Yes, if you are using an older generation of Bluetooth on one item, the second device can still pair and function.
FM transmitters allow you to play music or other audio from your phone with your car speakers by using a FM station. The station must be one that is not used in your area to avoid a static connection.