I have seen that in some scenarios word police is treated as a singular noun and in some other scenarios it is treated as a plural noun. I don't know the exact difference!

Can anybody provide few examples demonstrating the scenarios when the word police is treated as a singular and a plural noun?

Does it depends on the context only to treat it as plural or singular?

like consider these examples:

The police are blocking off the street where the accident occurred
A police officer is getting information from the neighbors.
The police department is at the corner of First and Main streets.
The New York police force has a special counter-terrorism squad
The police force is responsible for catching criminals

In the above examples, I can see the usage of 'Police' word as both singular and plural. I need to understand the exact concept when it will be as plural or singular?