Understanding Swiss Trains

Use the swiss rail site at sbb.ch/en to get an idea of travel times and point to point ticket prices.

Then tackle which (if any) train pass suits you best by doing a mini-spreadsheet.
You don't need to book tickets on most trains, but DO need seat reservations (small surcharge) for specialty trains like the Glacier Express.
Yes, there are regional/slower trains that make multiple stops, and Intercity, or faster, trains which use the same tracks.

You can mostly ignore the different companies. From the point of view of the traveller there is one system. On www.sbb.ch you can look up times and prices between any two points (down to street addresses) in Switzerland, and you can also purchase tickets.

Note that the SBB planner will normally always propose the fastest route from A to B, which may not be a scenic route. In order to force a certain route you can add a "via". Knowing a bit of geography helps here.

If you have a Swiss Travel Pass you have unlimeted travel on all public transit. You do not need to book anything, Just go to the station or bus stop, get on and ride. Tickets and passes are checked on board. A pass serves as a ticket.
When you buy a ticket or pass you pay to the "system", not an individual operator.

Now there are a handful of trains that require reservation. The main one is the Glacier Express, which requires a reservation. Another one is the Bernina Express, where you need reservation in the panoramic coaches, but not in the normal ones.
Reservations you buy from the operator, in this case Rhb -> www.rhb.ch

You can also reserve seats on the Golden Pass Express (www.gpx.ch). This is not required, but this summer will probably be a good idea.

Yes, there are lots of different train companies, with names shortened to acronyms like MOB, BOB, SOB, BLS, RhB, and so on. They are related to the history of the Swiss rail system and how it was built—-some have narrow-gauge lines, for example. But as WengenK says, you do not need to be concerned with that——they are all listed on SBB and you can buy tickets though the website or the app. It is only a few named “scenic trains” that need separate reservations, mainly for their special cars.

Those named trains may or may not have special “panoramic” cars with large windows, but the main feature is the route they take and the number of stops along the way (generally fewer than the regular regional trains that run on the same tracks). You can see the routes on this map of the Swiss railway system, where they are highlighted:

https://www.travelswitzerland.com/en/download/swiss-travel-system-map-en-2023/?wpdmdl=14294&masterkey=63a023c8da36c

For the specific trains you ask about:

For Zurich to Locarno, there is a direct train over the Gotthard Pass route, named the Gottardo Treno, but it is not a special “scenic train”, simply a regular regional train which requires no special tickets or reservations. Look for IR 46, which departs Zurich every 2 hours at xx:05 (8:05, 10:05, etc.).

From Locarno to Zermatt, you have two route options:

(1) you can go back on the Treno Gottardo as far as Göschenen and change there to a train going to nearby Andermatt. There you can catch the named Glacier Express if you wish (reservations required), or you can ride the regional train and change trains at Brig to reach Zermatt.

(2). You can take the scenic Centovalli train from Locarno east to Domodossola, anchge there to a train through the Simplon tunnel to Brig, an change there to the train to Zermatt.

From Zermatt to Montreux you also have a choice of routes, one going via the Lötschberg Basistunnel and Spiez, to the Golden Pass route, and the other through the Rhône Valley to Martigny and on to Montreux. I suggest you use that one, as you will be on the Golden Pass route from Montreux to the Berner Oberland.

As you noted, from Montreux to Interlaken Ost, gateway to the Berner Oberland, SBB tends to route you through Bern, or even back to Visp and through the deep tunnel to S;pier, as these are faster. But you can always choose to follow the Golden Pass route instead, using either the named Golden Pass train (reservations may be required for this, so check) or regular regional trains on the same tracks.

We just finished 10 days in Switzerland and bought the Swiss rail pass in Geneva for 15 days/first class. We took the Chocolate train out of Montreux and reserved seats. We also reserved seats on the Golden Pass express from Montreux to Lucerne. After taking the boat to Fluelen from Lucerne, we had reserved seats on the Gotthard Panorama Express ( a great way to get to Lugano). The ticket allowed us free access to castles, museums, funiculars, trams, buses, boats, and cable cars. The only one we had to pay for was in Lugano going up to Mt. Bri and it was discounted. When you are on board the train they will ask to see your Tavel pass for first class and then the seat reservation. Once we had the Swiss travel pass we booked everything online from our hotel room through the SBB site. We learned that crossing borders we needed a ticket once we hit the border for that country such as Italy, or leaving France. Also it was hard to figure out what part of the train was first class, but learned the monitors on the platforms tell you that, also the trains have numbers on the outside. It was the best experience, and we learned by doing.