Think of a car stopped with it's siren or horn or radio (playing the same note) turned on, it has a certain fixed frequency. Then the car moves towards you, it's siren stays exactly at the same frequency as when it was stopped , but this frequency is apparently higher only to you. This is because more waves are entering your ear per second than when the car was stopped. To the car driver, he/she does not notice any difference in frequency. So that's the source approaching the observer.
If the car stopped and you approach it, at high enough speeds, (in another car), you will still get the same effect, an apparent increase in frequency, but still only apparently higher to you, not the first car driver.
Last case, the car drives towards you and, at the same time , you drive away from the car. The apparent frequency you hear will depend on the difference in the two car,s speeds.