Could be a wrapper around WhatsApp Web.

It is, the matrix WhatsApp bridge uses a reverse engineered WhatsApp web library.

> A Matrix-WhatsApp puppeting bridge. Written in Go using the Rhymen/go-whatsapp implementation of the sigalor/whatsapp-web-reveng project.

https://matrix.org/docs/projects/bridge/mautrix-whatsapp

I'm sure it could be done that way, but if it violates TOS then I would be wary that the bridge will be unreliable.

I had that thought, too. That’s the reason that stopped me from implementing sth like that. I think a definitive solution to a cat-and-mice game would be an automated reverse-engineering of the chat apps.

YouTube-DL is successful despite it violating the ToS.

That's exactly what it does!