Hello vivianhorii
The ticket is correctly named Saver Day Pass, hence, yes, it is valid for the entire day on any public transport service in Switzerland. It is a day pass that allows travel at will. You do not need further reservations to use this ticket on the mainline services.
It is valid from Domodossola, you only need a ticket and seat reservation for the Milan to Domodossola stretch.
Please have your passport ready, the Saver Day Pass is only valid with your physical passport, national ID card, or driving licence.
Kind regards,
Roland