There is also the very informative special forum as community.eurail
You get a QR code that stays the same the whole day. You are supposed to add all trains used as they appear locally on screens/tables etc-this may or may not be checked. In general it is advised to only generate the code-in case of a pass with X days in 1 month, untill you are 123% sure you will use that train. THis is mostly for the X in 1-2 Month type, as on that day the code-and thus the passday used, cannot be extinguished anymore (think of strikes, falling sick etc)
I have used dozens of Interrails-always paper, till last year and used a 2 MO continuous ending mid may, my very first on mobile fone and was quite happy with using it and found no specific problems. It all worked fine and all inspectors also knew about it-and even in most cases staff at counters when needing to make REServations-in some 6 countries where it did not prove possible or easy to do that online.
For Swiss one has to note that the pass is NOT valid as extensive as the Swiss Pass or other local fares-some of the major touristy going up high in the sky/snowy mountains are not taking this cheap pass.
It is very easy indeed to change train used to other as planned before-once you get the hang of it, so dot not worry too much about that.