Hello Xuhoimogh98
There is no need to pre-purchase train tickets, you can also purchase it upon arrival at the SBB Travel Center.
Swiss trains are free-seating, there is no seat assignment with your ticket. And, Geneva-Airport is the departure station of your train, thus plenty of seats left. Same for Bern to Interlaken Ost; the train’s occupancy will be rather low during the day in the direction Interlaken Ost.
More on seat reservations:
https://community.sbb-tickets.cc/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Kind regards,
Roland