Can people order on WhatsApp by messaging me instead of using the page?

Ordering happens on the page, and WhatsApp carries the messages around it. After checkout the customer can tap a WhatsApp button that sends their own order summary to the hand-off number you set under Settings → WhatsApp connection & groups. Replies to any message land on your linked number like a normal chat, so a customer can still talk to you. A restaurant ordering system that takes orders inside a chat would need somebody to read and retype them, and that is the work this replaces. The WhatsApp tab in the docs explains the hand-off number.