Please kindly note, the ticket you are using is named Swiss Travel Pass. The SwissPass is used by residents.
The Swiss Travel Pass validity map gives you an excellent overview on its validity, see this website, in the Downloads box: Swiss Travel Pass. | SBB
To answer your question: travelling from Chamonix, you must have to get a ticket for the French stretch, from Chamonix to the border station at Le Châtelard-Frontière.
For the Centovalli railway, from Locarno to Domodossola, the STP grants - in theory - free travel, as both end points of the line connect with the Swiss rail network, and the line serves as shortcut between Locarno and Brig, hence it is therefore often used by Swiss locals, too. However, I said in theory, as with a free travel ticket, the conductor will still ask you for an EUR 1.50 “scenic supplement” per person. A justified small donation, to keep up the charming, and indeed scenic train services.
Have a nice travel.