Using a Swiss Travel Pass when starting in another country?

A Swiss Travel pass is valid from Chamonix Mont Blanc station to Montreux. To go to Freiburg im Breisgau, just show your pass when you buy the ticket and you will only be charged for the German portion. Same going to Zurich.

Be sure to have your pass in hand in Chamonix as I don't think they sell them there.

Marym, the Swiss Travel Pass is valid on all Swiss owned rail lines.
To be precise, it is valid on all rail lines shown in RED on this map: https://www.sbb.ch/content/dam/sbb/de/infotexte/uebersichtskarte-sts.pdf (plus the yellow bus routes and white boat routes)
As you can see, in a few cases this extends slightly over the border. It is not valid on non-Swiss lines (grey). On of these extra cases is the line from Martigny in Switzerland to Chamonix, but no further.

If you want to go to anywhere else outside Switzerland, the pass is valid to the last Swiss station, and you need to buy a ticket from that station to your final destination.
For your trip Bern to Freiburg im Breisgau, the last Swiss station is Basel Badischer Bahnhof, and you need to buy tickets from there to Freiburg im Breisgau. You must buy this ticket from DB (German Railways). If you already have this ticket, you do not need to get off the train at Basel Badischer Bahnhof.
Similarly the other way, you need a ticket from Freiburg im Breisgau to Basel Badischer Bahnhof.

Thank you so much for your help!

Chris F, is the train to Kleine Scheidegg included in the Swiss Travel Pass?

From the map you linked, I cannot tell. I am not sure if those are red dashes or red dots and what both of them mean. Thanks.

is the train to Kleine Scheidegg included in the Swiss Travel Pass?
From the map you linked, I cannot tell. I am not sure if those are red dashes or red dots and what both of them mean.

Continental, Those are red dots (. . . ), plus a "1" in a circle, which if you look in the key (top right) is 25% reduction with a Swiss Pass.
It is a solid line (Pass 100% valid) up to Wengen, because that is the last inhabited settlement so it is public transport. Above that it is a tourist railway with a reduction.

Dashes (- - - -) means the line is in a tunnel. Switzerland is 3-dimensional, you get some lines that go in tunnels under other lines with no connections.

Thank you!

If I choose to go to Kleine Scheidegg, can I buy a return ticket between Wengen & KS at the Luzern station ticket window (my hotel is in Luzern) or must I do it at the Wengen ticket window to get the 25% reduction? I mentioned Wengen because it is included in the Swiss Travel Pass.

Also, whichever station you recommend, would I also get the 25% reduction if I use a ticket machine?

Thank you.

If I choose to go to Kleine Scheidegg, can I buy a return ticket between Wengen & KS

You should be able to buy this ticket at any manned Swiss station, and from any Swiss railway ticket machine (though it might be non-obvious which option to select).
It's one train from Lauterbrunnen up to Kleine Scheidegg; having to get of at Wengen to buy a ticket, and then get on the next train, would be a pain.

Thank you very much. Yes it would be a pain to disembark from the train merely to buy a ticket. Relieved that it will not be necessary.