Please check the basics about the app here:

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

In particular the section "Getting ready to board the train”.

If that doesn't work for you, then please let us know in which step you are stuck.

Where can i see the QR code?
i already add my travel trip to the app (going to travel sunday) and have only 1 trip that need reservations (which i did) and all the other no need but will go via big site where i will probably need QR to get out

Yes you'll get a QR-code on your app. It should work in the Netherlands, however it may not be the case in other countries with ticket barriers : big stations in France, Italy and the UK mainly. In those places speak to staff nearby.

Where are you travelling ? Please note that you'll have to make some mandatory reservations, often on high-speed trains and cross-border routes. As it is peak season some of those trains can sell out a few days in advance (or more). Have a look at :

https://www.seat61.com/interrail-and-eurail-reservations.htm

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Optional seat reservations are also a good idea, for example on ICE trains in Germany.

There are always reservation-free slower alternatives (except Eurostar to the UK).