The one country pass for Switzerland is valid until Domodossola.

Also see this map for pass validity in Switzerland:

https://www.interrail.eu/content/dam/miscellaneous/Map%20of%20Switzerland_SBB.JPG

Or to Chiasso or Tirano further east. The Swiss pass is even also valid on the 100valli-Domod. toward Locarno. It is in fact for this transito for the Swiss that it is included.

Note that the direct EC treni dalle Svizzera per IT must be RES-cost 11€, you can avoid that by switching to Trenord regionale in these stazione. Or prebook ticket from those places to your casa.

The one country pass is not valid in Italy (in this case from Domodossola and south), these means no reservation is needed till Domodossola. From there if she goes somewhere in Italy she has to buy a normal ticket via Trenitalia. 

Hi to everybody and thank for the answers!
Yeah I’ll go in switzerland starting from tirano to get to st moritz with the bernina express and from there to zermatt with the glacier express. For the return, between domodossola and Milan I already planned to book a regional/intercity train with Trenitalia, I wanted to understand up to which city I could take advantage of the Swiss one country pass - in this case Domodossola.

Maybe if the 100valli is included I could also consider Brig - Domodossola - Locarno - Chiasso - Milan, but I'm afraid it's a bit complicated...

I definitely recommend you to take the Centovalli-Bahn. It's not complicated at all. You just have to go to the Domodossola FART station, which is next to the ordinary Domodossola station, to take the Centovalli-Bahn. 

The complicated part is not about taking the centovalli itself but all the trains I have to take that day. 

The goal of the day is to go back home in Milan from Zermatt, and the starting idea is to do Zermatt - Brig - Domodossola - Milan (3 trains)

Since in the morning I’m starting the journey from Zermatt, in case I’ll take the centovalli, my journey will change in Zermatt - Brig - Domodossola - Locarno - Chiasso - Milan (5 trains). 

I know centovalli is beautiful, but I wonder if 5 trains are too many trains to concentrate in one day to enjoy the ride

When will you be travelling? During November there are construction works on the Centovalli-Bahn. 

If you take the quickest connection between Zermatt and Milano the train ride is about 3,5 hours. If you go via Centovalli the ride is about 6 hours so I think that you will be fine taking the Centovalli-Bahn.

I’ll be traveling the 11th of December, I hope the centovalli we’ll be fine by that date

Thanks for the help!
Federica