For different types of google ads services, there are the following options available -
PPC Ads Services: In this option, you only pay if someone clicks on the ad.
Cost Per Impression Ads: Here, you can pay based on how many times ads are displayed on the Google network. This option you can use for a google display ads campaign.
Cost Per Engagement: This type of google ads service is offered for YouTube video ads where you will be charged based on whether the user completes a predefined task.