I want to book a trip from Paris to London and back and then to Amsterdam. How do I do that. Do I have to buy a Eurail pass and seat reservations?
I would use the France official train website (https://www.sncf-connect.com/app/en-en/home/search) to find & buy Paris/London/Amsterdam train tickets for their partner carriers (Eurostar & Thalys respectively). You don't need a Eurail pass for these trips.