I'm gonna assume you're not from the US given your lemmy instance. In the US most police radios transmit "in the clear", i.e., not encrypted. As such, anyone can buy a "police scanner", or a radio on the same band as police/first-responder frequencies.
In the internet age, there's websites (and apparently apps now too), that physically monitor these radio channels and stream them online. Hence, a "police scanner app". Hope that helps.
Having typed all that out, the only thing I ask is to let us know where you're from, please.
Yes, although many departments are switching to encrypted radios. It's a minor political issue, because it fundamentally means less oversight, but also in the event of a major riot or coordinated civil unrest you also might not want to broadcast patrol routed unencrypted.
Some time ago some American politician used the phrase unironically. Most speculated they were referring to Latin American countries, but since the politician was a republican and correcting themselves is anathema, we'll never know.
Anyway, it's become a favorite of mine to repeat this, tongue in cheek.