Learn Finnish: Oletteko te viidestään rakentaneet nämä isot marionetit? - Have the five of you built these large marionettes?

It means "a group of five" or in this context, "the five of you". It is formed by taking the numeral viisi, putting it in the elative case (viidestä), and adding a possessive suffix (here, -stään). This structure is used to express that an action is done exclusively by a group of a specific size.

Since the subject is te (you plural), why isn't the word viidestänne?

That is a great observation! Strictly grammatically, it should be viidestänne to match the second person plural. However, the third-person suffix -staan/-stään has become a fixed, fossilized ending in modern Finnish. It is now completely standard to use -staan/-stään for all persons, meaning whether it is "the five of us", "you", or "them" is simply inferred from the subject.

Why are the words nämä isot marionetit in the nominative plural case rather than the partitive?

They form the object of the sentence. Because the building process is finished and the sentence implies the whole group of marionettes is complete, it requires a total object. For plural words, the total object looks exactly like the nominative plural (ending in -t). If the building was still in progress, you would use the partitive plural: näitä isoja marionetteja.

Why is the verb split into Oletteko and rakentaneet?