How to buy a pass?
https://www.eurail.com/en/eurail-passes/global-pass
Seat reservations:
1) on their website https://www.eurail.com/en/book-reservations#/
2) at the main train stations
3) over phone
All details and methods for each country are described (scroll to bottom of the page):
https://www.interrail.eu/en/plan-your-trip/reservations/how-do-i-book-my-reservations
FAQ
https://interrailplanner.com/faq#:~:text=With%20an%20Interrail%20Global%20Pass,of%20the%20participating%20railway%20companies.