Newbie here—if train does not require reservation, do i need to do anything besides board the train?

Yes-you ALWAYS have to feed the trip actually made in the system-so that it generates a QR code that can be checked (if the conductor/country also has the equipment for that-in GB of course they have their own isolationist separate system). However, this code remains the same all day, even if you use 5-6-7 different trains. THis is (people wonder why) to in the final end after many statistics all railways get more or less their fair share of revenue-passes tend not to be sold in places where they are used the most!