Can I have a food ordering app for my restaurant without paying somebody to build one?

You do not need an app. Your ordering page works on every phone in a browser, and there is nothing to install on either side. A food ordering app for restaurants has to be downloaded before the first order, and most customers never do it. A link on Instagram, a QR card on the counter and the link in every WhatsApp message all open the same page. The Getting started tab in the docs covers sharing your link.