Learn Farsi: ثروتمندها و فقیرها در این جامعه زندگی می‌کنند. - The rich and the poor live in this society.

In Farsi, to talk about a whole group of people characterized by an adjective, you take the adjective and add the regular plural marker -ها. So ثروتمند (rich) becomes ثروتمندها (the rich ones) and فقیر (poor) becomes فقیرها (the poor ones). If you left off the -ها, it would just be a dangling adjective.

How exactly is the word "ثروتمندها" built?

It comes from the noun ثروت (wealth) plus the suffix -مند, which means "possessing" or "having". This creates the adjective ثروتمند (wealthy/rich). Finally, adding -ها nominalizes it into the plural noun ثروتمندها (the wealthy ones).

How is the "و" (and) between these two groups pronounced in everyday speech?

While written as و (formally pronounced "va"), in everyday spoken Farsi it drops to a short "o" sound. Because ثروتمندها ends in a vowel (the "a" sound of -ها), a connecting "y" sound naturally slips in, making it sound like servatmand-hâ-yo faghir-hâ.

Why is the verb "زندگی می‌کنند" plural here?

The subject of the sentence is a compound made of two plural nouns: ثروتمندها و فقیرها (the rich and the poor). Because these subjects are animate (they refer to actual people rather than objects), Farsi grammar requires the verb to agree in number, so it takes the third-person plural ending .