Hello there Mario2022,
Thanks for posting here on the Apple Support Communities! This is a place where other Apple users can submit questions or respond to others to help lend a hand. Let's take a closer look as we do our best to provide some additional insight.
Since WhatsApp is not an Apple application, they may sync your messages and documents with their own servers. This is typically done with an account directly with WhatsApp. In some cases your messages and documents may be synced through that account.
Otherwise, you can use these steps to locate their contact info: How to contact an app developer
We hope this helps get you pointed in the right direction.
Take care.