We might say, "the police" to refer to all of them everywhere, or to all of them in one office, or even to a group of them "over there" or whatever. We might also say "the cops". "Cop" is maybe a little less formal. Also, I think it has a very slight negative connotation, as of someone who does not like the police.
Moreover, in English you cannot say, "a police". You can say, "a police officer". Also, you can say, "a cop", as in, "Are you a cop?"
If someone works at a police station as a police officer, we would say, "He is a police officer," or maybe, "he is a cop". But again, saying "cop" does not sound as respectful, at least to me.
"He works for the police." "He works for the cops." The second sentence has the same negative connotation, but it is correct. However, saying "works for the police" doesn't necessarily mean one IS a police officer. For that, one would say, "he is a police officer."
"Someone in my family is a police officer/is a cop." "By brother is a cop." "My sister is a police officer." "My uncle works for the police." (He might be an informant.)