What are the key differences between using the official WhatsApp Business API and `whatsapp-web.js` for multi-device Node.js applications?

The official WhatsApp Business API is a paid service designed for high-volume, business-critical messaging. It requires approval from WhatsApp and offers features like template messages and analytics. `whatsapp-web.js`, on the other hand, is an unofficial, reverse-engineered API. It's free to use but is more prone to breaking changes as WhatsApp updates its web client. Multi-device support is natively built into the Business API, while `whatsapp-web.js` requires specific configuration and session management. The Business API is more reliable for production, while `whatsapp-web.js` is suitable for prototyping or low-volume personal use.