Can I run the whole thing from one Android phone: orders, the counter and stock?

Yes, with two things open. The dashboard works in the phone's browser: the orders board, the menu, stock and every report. The POS is a separate app that pairs with a six-letter code from Settings → The POS, and it runs on an ordinary Android phone as well as a tablet. Staff sign in on it with a four-digit PIN. A single phone doing both is fine for a stall or a small counter, and you can add a tablet later without redoing anything. The Getting started tab in the docs lists the first day in order.