It’s not, unless you know the keys.

Keys are created by the software/app made by the service provider, like WhatsApp / Meta or Google. How is the key created, and is a copy sent back to WhatsApp? “Securely” and “No” they claim, and you just have to trust them.
That can change if WhatsApp need to comply with new laws.

Signal is a bit different because of the app is fully open source, so the code can be audited to verify the integrity of the encryption. They would still need to comply with laws or exit that market, but whatever they do would be 100% transparent.