Reservation service is not connected with the app. You show yoir reservation on PDF on your Phone or printing it. Sometimes they send it by postal mail. It depends from the train company. 

If you want, you can post the exact route with the date, so we can give you some advices to book.

There is no connection between the app and the Interrail website. The app will be your ticket and that is separated from from the reservations that you either make at the Interrail website or elsewhere. 

Here you can read about various ways to make reservations. 

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Thank you!