A lot of my regulars pay with a payment app on their phone. Can I add that as a way to pay on the POS?

Yes. Open Settings → The POS and go to How people pay. Fill What is it called with the name your staff should see on the button, then decide the two ticks: Opens the cash drawer, and Counts as cash in the drawer. For a payment app leave both unticked, because the money goes to your bank and never into the tray. The Z report then shows that button's takings under its own name, and your drawer count stays honest. A restaurant POS system should follow how your customers actually pay, and that is what this list is for. The POS and the counter tab in the docs has a guide called Set up how people pay.