Impersonal Se vs. Passive Se
Impersonal expressions don’t reference a specific person. Rather, they make reference to people in general and are used to make general statements or questions about what “they,” “one,” or “you” do. That is what makes them impersonal!
You can use impersonal expressions to find out how things are done according to custom, rule, or general consensus. They are also helpful for asking for directions, finding out how to say things properly, asking what the rules are in a specific place, making general statements, and so on.
Impersonal se constructions are always formed with se + a third person singular verb.
The passive voice is generally used to talk about something that happened to an object without saying who or what carried out the action. Only transitive verbs (verbs that require a direct object) are used in passive se constructions. It's very common to use the passive se to talk about inanimate (non-living) nouns. It is not common to refer to people using the passive se.
Passive se constructions can be formed with se + a third person singular verb or a a third person plural verb, depending on what is being discussed.
Se vende carro usado. (Used car for sale.)
Se venden carros usados. (Used cars for sale.)