Does user privacy when using WhatsApp Web (https://web.whatsapp.com) differ substantially from using WhatsApp on Android? WhatsApp on Android has end-to-end encryption and (optional) encrypted backups. If I use WhatsApp Web, will Meta be able to see the contents of my WhatsApp messages?
Just as much as they would be able to see the contents of your messages when you use the app. The answer to this question depends on how much you trust Meta to stick to their claims. As others have already said here, use Signal if you are looking for a private messenger.