Booking site for trains

You should be able to book it all on the Swiss Train site SBB. https://www.sbb.ch/en

Train tickets you can buy on www.sbb.ch
That is the one-stop-shop for all public transit.

However on some trains you need a reservation on top of your ticket. And the reservation you need to get directly from the operator. The only train where reservation is compulsory is the Glacier Express. On the Bernina Express it is only compulsory in the Panoramic cars. Reservations you can get from www.rhb.ch

Tickets and reservations are separate, because tickets are for a route, not a train. You cannot rally buy a "train ticket" in Switzerland. To travel on the Berinana Express for example you would buy a ticket that covers the Bernina route, and separately a reservation for a seat in the particular service you intend to take.

And there are multiple options for tickets. If you are for example travelling Luzern - Chur - Tirano, with the Bernina Express only a part of the trip you would buy a ticket Luzern - Tirano, or you would get a Day Pass, or maybe you are doing enough travelling to warrant getting a Swiss Travel Pass. To that you then add your reservation. It is not necessary to already have a ticket even to get a reservation. So if you are still pondering wether to get for example a Swiss Travel Pass you can already make the reservations, and give yourself some time to figure out if you want a pass, or a discount card, or just normal tickets.

The Golden Pass route does not require reservations, but on the Montreux - Interlaken section they are a good idea. These you get on www.gpx.swiss. On the Interlaken - Luzern part I would not bother with reservations.

Note also that there is no need to buy train tickets in advance. Most people buy their tickets on the day of travel itself.

Tickets and reservations are separate, because tickets are for a route, not a train. You cannot buy a train ticket in Switzerland.

That second sentence is confusing to many people. Of course you can buy train tickets in Switzerland—-the next paragraph goes on to explain the different types of tickets one can buy. I will add that you can buy these tickets online, from a ticket machine, or from an agent at the train station. (Maybe not all types from all sources).

I believe the intent of that statement is to point out that you cannot buy a ticket that includes a seat reservation (like one would, say, for a Frecciarossa train in Italy). The seat reservation on a particular train is always separate (and good only for the reserved train). And it costs extra.

The exception would be for tickets on cross-border trains (i.e., traveling from Zurich to Milan on a EuroCity train). Then the seat reservation is included in your purchase.

When I say that there actually are no "train tickets" I mean quite literally. You cannot purchase a piece of paper that says "train ticket". The reason is that public transit is fully integrated, and there is one single national ticketing system. And even though you buy these tickets through SBB usually you are not in fact buying them from SBB. The money goes to a central organisation, that then distributes it to the different public transport companies.

That is also the reason why tickets and reservation are separate. I you buy a reservation for the BEX it goes to the RhB. The ticket revenue however does not go directly to the railway.

So you buy a ticket from one place to another, and the ticket is good for whatever modes of transport are needed to get there. You can buy a ticket from a small rural bus stop in the Valais to a request stop on a railway line in Graubunden.
When you buy a ticket you buy a travel entitlement, not a place in a vehicle.

Tickets are for travel. They are not for a train, or a bus. If you buy a ticket Zurich airport to Zurich Hbf it is good for any train, tram or bus that you choose to take advantage of on the way...