If you buy a pass then that is your ticket. You need to register every train you use in the app (mobile pass) or on the pass cover (paper pass). Also see this page.

For trains with mandatory reservations, you need to book reservations too.

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you, after having looked at the guide, have questions about how to make specific reservations, please create a new topic with your travel details (departure date, time and route) and you will get advice.