Seat reservations are not mandatory in Switzerland, you can use your pass (Swiss Travel Pass) from Interlaken to the border station, Domodossola or Chiasso, depending on your route.
From the border station to Venice, you should buy the reservations on trenitalia.com, according the rules of your Trenitalia Pass.
An important information about Trenitalia Passes:
A “trip” is one trip on one train regardless of length, not one journey! If you connect and change trains, that counts as 2 trips; plan your itineraries accordingly.
https://www.reddit.com/r/Interrail/comments/16kztp4/my_trenitalia_pass_experience_executive_class_is/?rdt=34422
Eurail Passes could be better as Swiss and Italian passes, but mandatory seat reservations in Italy are not free of charges:
https://www.eurail.com/en
Following routes are included in Eurail passes in Switzerland:
https://holidaystoswitzerland.com/wp-content/uploads/2024/03/Eurail-Global-Pass-Switzerland-coverage-2024.pdf