What is best option to buy train tickets

We booked train from Munich to Salzburg

I hope it was a Bayern Ticket.
How many in your family and ages?
When are you going?

Not sure but it was from behn site. 3 adults going (1 is under age 20). Going from May 15 to 30th

May 22, 3 tickets Salzburg to Interlaken Ost dep 9:56 am costs 164 EUR total, 7 1/2 hour travel time from OEBB
May 23-25 2 adult 3-day Berner Oberland Passes at 240 CHF each and 1 Youth 3-day Berner Oberland Pass at 168 CHF
May 26 3 tickets Interlaken Ost to Rome Termini dep 7;40 am cost 159 EUR total, 6 2/3 hour travel time from Trenitalia

May 22 and May 26 tickets are nonrefundable train specific tickets bought now. Like airline tickets.

Thank you. Where do I buy these? And how do I see ‘transfer’ time with our luggage is ok or not?
Exact dates-
May 21 - Salzburg to Interlakan west
May 25 - Interlakan west to Roma termini

Tickets you buy from www.oebb.at, www.sbb.ch and www.trenitalia.com depending on where your trip starts. The Berner Oberland pass you buy on www.bls.ch or at the station once you are there.
Transfer times are planned under the assumption that you are a old, nearly blind person with lots of luggage. So for normal healthy adults that should be find. Unless you have packed as if you are moving countries, which unfortunately some tourists do...

Thank you very much. I’ll book

Transfer times are planned under the assumption that you are a old, nearly blind person with lots of luggage.

I don't quite agree. The major assumption is that the trains are on time, which is not always the case. OeBB sometimes offered me connections having a transfer time of less than 10 minutes, an then the train was 6 minutes late. The connecting train will not wait, i.e., the train staff is not allowed to wait - as it turned out recently due to a complaint about one minute -, because the time slots given on the tracks are so dense that a delayed train causes more havoc than a fed-up passenger.

Where do I buy these?
Salzburg to Interlaken: https://shop.oebbtickets.at/en/ticket
Berner Oberland Pass: https://www.berneseoberlandpass.ch/
Interlaken to Rome: https://www.trenitalia.com/en.html
Trenitalia insists that you use proper Italian city names, so destination is "Roma Termini"

When you shop for trains, the transfer times will be displayed. Swiss transfer times are usually short, but always doable.

Or if you'd rather, use one of the train ticket companies mentioned by the Man in Seat 61

I don't quite agree. The major assumption is that the trains are on
time, which is not always the case. OeBB sometimes offered me
connections having a transfer time of less than 10 minutes, an then
the train was 6 minutes late.

The reason ÖBB offered you a connection of 10 minutes is because you were changing at a station where the MCT was less. MCTs are typically in the 3 to 5 minute range in well run train networks. So with a 5 minute connection you already have quite a margin. In Vienna HB you would never need more than 4 minutes to change main line trains, and maybe 5 if changing to the S-Bahn.

And the railways do have margin to make trains wait for connections, and train conductors will not dispatch their train if there are still people boarding. Don't forget that if you are at the door of the train one second before departure you are on time.

So yes, these connections work. In fact, a railway forcing a connection of more than 10 minutes is wasting the travellers time. This is especially callous when a lot of tax payer money has been invested in speeding trains up, only then to be lost through railways not being bothered getting schedules right (looking at you, SNCF and RENFE here...)

MCTs are typically in the 3 to 5 minute range in well run train networks.

The key words are "well run". I guess you are used to SBB in Switzerland, for which I would fully agree.

ÖBB is pretty well run too. The important thing is to build stations in a way that you can move passengers fast. The new Vienna Main Station does that rather well. Also ÖBB, just like SBB, does add some buffer to their schedule, so trains can wait a few minutes for each other.

Thank you Sam and everyone. I booked rail tickets and purchased overland pass for Swiss.
I did not get any unlimited pass for Salzburg or Rome. I’ll ask other question in new thread.