By accessing the device. It is not only E2E encrypted, WhatsApp servers are only a middleman, as soon as they arrive (double-check) they will be deleted from the server. Undelivered messages are retained up to 30 days, according to their privacy policy:
<<Your Messages.
We do not retain your messages in the ordinary course of providing our Services to you. Instead, your messages are stored on your device and not typically stored on our servers. Once your messages are delivered, they are deleted from our servers. The following scenarios describe circumstances where we may store your messages in the course of delivering them:
Undelivered Messages. If a message cannot be delivered immediately (for example, if the recipient is offline), we keep it in encrypted form on our servers for up to 30 days as we try to deliver it. If a message is still undelivered after 30 days, we delete it.
Media Forwarding. When a user forwards media within a message, we store that media temporarily in encrypted form on our servers to aid in more efficient delivery of additional forwards.
We offer end-to-end encryption for our Services. End-to-end encryption means that your messages are encrypted to protect against us and third parties from reading them. Learn more about end-to-end encryption and how businesses communicate with you on WhatsApp.>>
> End-to-end encryption means that your messages are encrypted to protect against us
So, they say the protection is there once the encryption has been applied. They say nothing about what happens to the content before or after that on the end user's devices. That handling is however covered by other legitimate use clauses in the privacy statement. This covers keyword scanning for targetted ads (so a defence lawyer will say at some point.)
<<Your Messages.
We do not retain your messages in the ordinary course of providing our Services to you. Instead, your messages are stored on your device and not typically stored on our servers. Once your messages are delivered, they are deleted from our servers. The following scenarios describe circumstances where we may store your messages in the course of delivering them:
Undelivered Messages. If a message cannot be delivered immediately (for example, if the recipient is offline), we keep it in encrypted form on our servers for up to 30 days as we try to deliver it. If a message is still undelivered after 30 days, we delete it.
Media Forwarding. When a user forwards media within a message, we store that media temporarily in encrypted form on our servers to aid in more efficient delivery of additional forwards.
We offer end-to-end encryption for our Services. End-to-end encryption means that your messages are encrypted to protect against us and third parties from reading them. Learn more about end-to-end encryption and how businesses communicate with you on WhatsApp.>>
https://www.whatsapp.com/legal/privacy-policy/?lang=en
Assuming they are compliant with their privacy policy, they don’t even have the messages.