Do I sign up separately for WhatsApp, stock and the POS, or is it one account?

One account. Signing up gives you the ordering page, the dashboard and every feature your plan includes, and WhatsApp is linked from inside it by scanning a QR code under Settings → WhatsApp. Stock, recipes, suppliers, staff and reports are in every plan with nothing to switch on. The POS is the one thing bought separately, as a yearly add-on from Settings → Plan & billing, and it keeps working during the free trial so you can set it up first. There is no second login, no second bill for stock, and no separate restaurant software to connect.