I would keep them seperate, you never know if a phone breaks or get’s stolen. Ticket gates are absolutely not common, mostly in bigger stations in the UK, France, Spain and the Netherlands. Mostly the QR-code doesn’t even work on the scanners, so you should find a member of staff to open it for you (usually there’s always staff and the qr code works in the Netherlands to open gates).
You cannot reserve trips via the rail planner app. You register your journeys there. See the tab “My Trip” as your rough schedule, or your train diary. Tapping the grey ledger in “My trip” transfers a specific train to the pass. Generating a QR-code. This QR-code is your ticket enabling you to board but this is NOT a seat reservation or a “booking”. You find this QR-code and your registered journeys under the tab “My Pass”.
Please only do this activation only shortly before taking the first train of the day, otherwise you risk losing travel days when something unexpected happens. No need to do this ages in advance. You can do it in advance, to test the app and it’s functionalities. But make sure you disable everything and de-activate it, in orther not to lose precious travel days. :)
Seat reservations are completely seperate from the app/pass. Best use other websites to get these. The community has a handy overview of where to get them (other than Interrail/Eurail reservation service, as it charges quite a lot extra). See link below.
Only a few trains have mandatory reservations, usually long-distance trains, high-speed trains or night trains with a proper bed. Most overseas tourists only use these between the bigger, touristic cities, which complicates journeys a lot and adding quite a bit of an extra cost.
Some trains have optional reservations, mostly found on high speed or intercity services in the UK and central Europe (Germany, Austria, Switzerland, Czechia,...)
The rest of the trains (more than 90% of all trains in Europe) are all trains without any form of reservations. Just register your train in the app, and hop on and enjoy the ride.
Feel free to get back to us, with specific questions about routes, alternatives to avoid trains with sometimes very expensive mandatory reservations or other rail insights :)