An order already holds everything you need to invoice it, so you never type it twice.
The invoice is linked to the order: it appears in the order's Invoices card and in the Invoices column of the orders list, and its payments in the Invoice payment list. You need the Invoice module and full access to invoices.
With Show payment option after submitting the order on, orders placed by clients get their invoice automatically. See Get paid when an order is placed.
In the Order info menu, Create Estimate opens a new estimate with the client, taxes, discount and all items of the order. Useful when a client orders but needs a formal quote first. See What are Estimates in NIZU?
In the same menu, Create project opens the project form for the client. Once saved, the project is linked to the order and shown in its Order info card. Use it for orders that need work: an installation, a custom build, a set of services.