Learn Italian: Finita la riunione, chi lavora bene, chi si distrae. - Once the meeting is over, some work well, some get distracted.

This is a distributive correlative used to contrast subgroups, translating to "some... some..." or "some... others...". It paints a picture of a scattered group doing different things simultaneously. While alcuni ... altri means the exact same thing, chi ... chi is often preferred for a punchy, proverbial, or highly contrasting tone.

Why are the verbs lavora and si distrae in the singular when they refer to multiple people?

The pronoun chi is always grammatically singular in Italian. Even when translating to the plural "some" in English, you must conjugate the verbs following chi in the third-person singular. If you used alcuni ... altri instead, the verbs would have to be plural (lavorano, si distraggono).

What grammar structure is Finita la riunione?

This is an absolute participle construction, which is a highly common and concise way to say "After the meeting finishes" (Dopo che la riunione finisce). Notice that the past participle finita must agree in gender and number with the noun that follows it, which is the feminine singular la riunione.

Since finita is a past participle, why does the English translation say "Once the meeting IS over"?