A secure messaging and calling app, available on phones all over the world.
When registering, you are unable to sign up for WhatsApp without a phone number.
Even though they do not retain the content of the message, they collect location (even without permissions through IPs) and who and when the messages and/or calls take place.
If you chose to upload/give access to your address book. This goes both ways, if one of your contacts chooses to do the same.
If you do any purchases through WhatsApp and/or Facebook Pay.
If you are in contact with customer support, and you provide them with screenshots and "any other information you deem helpful"
Via diagnostics, performance information, if you interact with a business (when, how often and duration of interaction), log files, crashes, website visits, how often you message and/or call, status updates, group calling and messaging, payments, online-status and when you last updated your "about" section.
Via browsers, phones and other services; hardware model, operating system, battery level, signal strength, app version, browser information, mobile network, connection information, language, time zone, IP address and device identifiers.
Either from your devices GPS if you've given permission, else via your IP address and phone number.
If someone shares an address book with your phone number inside, all metadata associated with that number will be collected, i.e. real name, address, birthday etc.
A business on WhatsApp can choose you share their interaction with you to WhatsApp.
Through your interaction with their websites and applications, also from other Facebook products/companies.