Hello Superque16
Thank you for your question.
If you have the Swiss Travel Pass, you can travel fro free by train, bus and boat, you do not need to buy additional tickets.
If you wish to purchase a seat reservation, you can do so in the SBB Mobile App or on our webiste: Seat reservations in trains and dining cars | SBB
Concerning privat railways or mountain cable car not completely covered by the Swiss Travel Pass, you can buy a ticket on their website with the possibility of enterning “Swiss Travel Pass” as a reduction.
Please call our colleagues at teh SBB Contact Center (0041 848 44 66 88, CHF 0.08/min) for more information.
I wish you a pleasant day.
EleonoraS