Learn Italian: Dovete fare i compiti. - You all have to do the homework.

Because dovere is a modal verb. When you use a modal verb in Italian to say you 'have to' do something, only the first verb changes to match the subject (dovete). The second verb always stays in its infinitive or dictionary form (fare).

Where is the word for 'you all' in this sentence?

It is built right into the verb dovete. Because Italian verb endings change for every person, the ending tells you exactly who is doing the action. The pronoun voi (you all) is usually dropped unless you really need to emphasize it.

Could this sentence also mean 'You all must do the homework'?

Yes! The verb dovere means both 'must' and 'to have to'. Italian doesn't make a grammatical distinction between the two like English sometimes does.

Why is 'homework' plural (i compiti) in Italian?

In English, 'homework' is treated as a singular, uncountable concept. But in Italian, un compito is a single task or assignment. When teachers give you schoolwork, it is usually multiple tasks, so Italians use the plural i compiti.