Learn Finnish: Tämä pyramidi olisi tehty betonista. - This pyramid would have been made out of concrete.

It is formed by combining olisi (the active third-person singular conditional of the verb 'to be') with tehty (the passive past participle of tehdä, 'to make'). Together, olisi tehty means 'would have been made'.

Since this is a passive sentence, shouldn't the helping verb be 'oltaisiin' instead of 'olisi'?

No, and this is a very common trap! In the Finnish perfect passive (and pluperfect passive), only the main verb is in the passive participle form (tehty). The helping verb is always in the active third-person singular (on, oli, or in this case, olisi).

Why is 'Tämä pyramidi' in its basic nominative form here?

In passive sentences, the object (the thing being acted upon) is put in the nominative case if it is a whole, countable thing. Because we are talking about one complete pyramid being made, it takes the nominative Tämä pyramidi rather than the partitive.

Why does the word for concrete ('betoni') have the '-sta' ending here?

The elative case ending -sta/-stä (usually meaning 'out of' or 'from') is used in Finnish to indicate the material that something is made from. So betonista directly translates to 'out of concrete'.