Reservations are not needed in Switzerland, they're a complete waste of money and not a thing at all. I've lived here all my life and never made one :)

Add the train to the app before boarding (in order to get the QR-code) and that's it. Board and sit wherever you want.

Trains are very frequent and have lots of capacity. People are lazy and board the train in the middle : if you'd like emptier carriages, go to the train ends.

Reservations are only mandatory on the Glacier Express (49 CHF) and on the panorama carriages of the Bernina Express (24 CHF). Those can easily be avoided by taking hourly regional trains on the same route or the reservation-free carriages at the front of the Bernina Express. Those other options are actually better : less crowded and pull-down windows :)

Do not hesitate for further questions.