The app uses the (i)phone OS’s cloud storage APIs to write to the backup folder, meta’s servers don’t have access to any credentials.
For Android I currently can’t check, but it’s obvious from their FAQs that they have the app upload to Google‘s servers even if they don’t use the OS APIs there:
https://faq.whatsapp.com/481135090640375/?cms_platform=andro...They have indirect control over the user’s backup folder via the app, but meta would need to distribute a malicious update to everyone that causes the user’s apps to download the backup and send it to meta, at which point they could just skip trying to access the backup and directly upload the chats from the app.
It’s impressive how much misinfo you’re spreading.
Edit: Meta’s actions over the years clearly show that they don’t want to know the contents of your messages. Not knowing their contents means, for example, that they don’t have to run scanners to detect illegal content (but users can report messages).
They benefit from making WhatsApp a secure platform, as it allows them to collect everyone’s metadata, which apparently has lots of value to them.