Is there any sample project how we can create an order entry (with composition entity) UI in Polymer?
The sample-portal application (which is also available from the Samples tab of CUBA Studio) contains polymer UI module and the data model as you described.
Is that sample app generated using any template in the CUBA studio?
Studio screens scaffolding is used as the starter point. After that, the screens were manually tuned to achieve the final result, which you can see in the demo project.