I had this before I upgraded my stereo. You plug in this device to the DC power in your car, then you run an AUX from the ROAV to the Aux jack in the car. You connect your phone to the ROAV and it just starts playing every time you get into the car. Make sure you download the app though so you can shut off the guys voice that tells you it connects. After you get all the settings exactly how you want them you can delete the app.
You can also put a flash drive in it and it will stream music off the USB. I know you are thinking "Well, I will just grab one of the 15$ ones instead of paying 25$." DONT DO IT! My ex-GF bought 3 of the cheap ones totaling 55$. They had horrible sound, they would fry themselves, and the buttons quit working on them. Then she "Broke down" and spent $25 and this has worked flawlessly since.
After I upgraded my Head unit I gave this to my mom and filled a flash drive with her favorite music. So it has been going strong for us for 2 years now and it doesn't look like its gonna slow down.
EDIT: It looks like they have a newer version Amazon.com: Anker Roav Bluetooth Car Adapter and Car Charger, Power IQ 3.0 Type C PD, Bluetooth FM Transmitter for Car, Wireless Calling with Bluetooth 5.0, Noise Cancellation -T2: Electronics Go with whatever you would like, but bluetooth 5.0 is a nice upgrade from bluetooth 4.2. They will both work great, but 5.0 is a little quicker and a little higher fidelity. It just depends if you want to save 5 bucks.