When desktop visitors on your site click the Save to Google Pay button, a lightbox appears with a preview of their Pass. Here, they can confirm the save. After confirmation, the Pass appears in their Google Pay app. The user can switch which Google Account to associate with their offer by clicking their profile picture at the top right corner of the rollover frame.
On mobile, if a user with Google Pay installed clicks the Save to Google Pay button, the user is taken to the Google Pay app to complete the save. Otherwise, the button takes the user to a mobile web page to complete the save.
For more information, see Save to Google Pay.