Broadcast lists
Built into WhatsApp, broadcast lists send an individual message to each contact on the list. They are free and simple, but limited: up to 256 contacts per list, and recipients only get the message if they saved your number. There is no import, no real personalization at scale, and no reporting.
WhatsApp Business API (BSP)
The official API is built for scale and compliance. It supports high volume, approved message templates, and integrations, usually through a Business Solution Provider. The trade-offs: an approval process, per-message or conversation pricing, template pre-approval, and a strict opt-in requirement. It fits larger businesses with budget and a compliance process.
Chrome extension (e.g. Beamiro)
An extension runs on WhatsApp Web and sends from your existing number — no API approval, no per-message fees. You import a list, personalize with variables and spintax, pace sends with delays, and go. It is the simplest, cheapest route for individuals and small teams doing personalized outreach.