Learn Farsi: زمانی که پلیس رسید، جمعیت پراکنده شدند. - The crowd scattered when the police arrived.

Because جمعیت is a collective noun referring to a group of people. In Farsi, collective nouns involving humans (like a crowd, team, or family) usually take a 3rd-person plural verb to emphasize the individuals within the group acting together.

If collective nouns take plural verbs, why does پلیس (police) take the singular verb رسید in the first half of the sentence?

Excellent observation! While human collective nouns often take plural verbs, they can also take singular verbs when the speaker is thinking of the group as a single, unified entity. Here, the police are viewed as one institution arriving (رسید), whereas the crowd is viewed as many individuals scattering (شدند).

Is زمانی که different from the word وقتی (when) that we learned earlier?

They mean exactly the same thing and are used the same way to connect two clauses. زمانی که literally means "the time that" and sounds just a tiny bit more formal or written than the everyday وقتی, but you will hear both frequently in spoken Farsi.

How does the verb پراکنده شدند work here?

This is a compound verb. It combines the adjective پراکنده (scattered or dispersed) with the verb شدن (to become). So پراکنده شدند literally means "they became scattered."