Hello Rirushouf99
Firstly, welcome to the SBB Community.
When purchasing a train ticket in Switzerland, the train operator makes the promise to to offer transportation from point A to point B. However, there is no promise for an available seat as train tickets in Switzerland do not include a seat. Our train are walk-in; with a regular ticket you can use any train serving along your route. You simply get on board the train and find yourself an available seat.
An additional seat reservation is possible but not recommended when arriving by air. What if your flight is late or the baggage delivery takes longer than expected; right, the train with your reserved seats will have left.
You can purchase a Saver Day Pass or a point-to-point ticket (no Supersaver tickets) already now or you can wait an purchase them upon arrival. The Saver Day Pass is only available in advance, not on the day of travel.
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
Kind regards,
Roland