Police will sometimes attend when an ambulance is called.
However not all ambulance requests require the police to attend or show up along with them.
But if there's violence involved or a crime scene where someone is injured due to a crime such as a stabbing, shooting etc then the police will arrive first, secure the scene and then allow the paramedics to enter the crime scene to do what they need to do.
The paramedics won't go into the crime scene even if someone is injured badly until the scene has been secured.
For most ambulance calls the ambulance only shows up.
However if someone dies then the police will show up to make a report and note down that it was not a crime or murder when the person died.
Most times though when you call an ambulance it will be a first responder or just the ambulance.
However if the police can get to the location faster than the ambulance then the police will usually show up and render aid as much as they can while they wait on the ambulance.
Or in some cases the fire department will show up as they are also trained medically.