Contact Manager is a Gmail Add-on that integrate Google Contacts with Gmail. For showing email senders and recipients as contact cards, it need to read current email thread for getting senders and recipients email address and then get user’s contacts by email address. User can update or delete contacts manually. If senders or recipients are not in contacts, user can create new contacts with their email address. These operations are trigger by user, not by add-on. Your contacts are only used for showing contact cards to you. It is not used for other purpose and does not share your data to anyone. The add-on does not store your contacts externally, it directly get contacts from Google, like a contact client app. For search contacts by their emails, the add-on need to cache the pair of contact’s email and contact’s id(resourceName). Because Google apps script doesn’t support Google people api now, the add-on need to use UrlFetchApp to call people api.