Hello Rpita
From Domodossola to Switzerland or from Switzerland to Domodossola, seat reservation is not mandatory. You can usually travel without a seat. If the train is very crowded, passengers without a seat reservation may be asked to get off and travel on a regional train. From Domodossola to Brig you do not need an additional train ticket if you have the Swiss Travel Pass.
I wish you a nice day.
Kind regards,
FatimaH