In 70-level events, you have to complete 7 stages of 10 levels each. However, your task is not simply to complete the levels in each event stage, but also to collect skill stars. You can get up to 3 stars for completing each level. The more stars you collect during the event, the greater the reward that you can claim on the event screen.
Please note that, as in season events, initially only rewards on the left side are available to you. To obtain the additional special rewards, you will need to purchase the Star Pass. The pass unlocks access to all the rewards on the right side, provided that you have collected enough stars to unlock and claim them.