ashishjain12 Between Geneva and Domodossola you have two kind of trains:
— A few Eurocity (EC), without change in Brig. You can reserve seats on sbb.ch.
— Interregio (IR), with a change in Brig. It is not possible to reserve seats on these trains.
It depends from the time you leave Geneva. It is not necessary to book a ticket with your Swiss Travel Pass for these lines, until you want reserved seats.
Between Domodossola and Locarno, you can reserve seats here, it is not mandatory but recommended:
https://www.vigezzinacentovalli.com/en/