Not 100%, because I never found a confirmation of it. But it uses omemo that is a xmpp extension. WhatsApp I'm sure it does.
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WhatsApp uses a customized version of the open standard Extensible messaging and presence protocol (XMPP).[210] Upon installation, it creates a user account using the user's phone number as the username (Jabber ID: [phone number]@s.whatsapp.net).
WhatsApp - Wikipedia
And by how signal collaborated with WhatsApp encryption I would say it's very likely signal is xmpp also. But no confirmation so far.
🤣😁😂