How to create an order form where multiple items and their quantities can be added?
Why are you choosing this approach if you are new to Power Apps?
Regarding your requirements, when you want to add a new order line you can popup a modal box (essentially entry fields in a container) enter the new order line, update the collection and refresh the display. A Delete button/icon would be on every line and just remove that row from the collection.
You could also use this tutorial to see if it would satisfy your requirements and would probably be easier to modify