Hello tesabcd
Good day, first.
In Switzerland, regular tickets are good for a route, not for a particular train.
Therefore, the ticket you are about to purchase is good for any train serving the route that day, not only the one you are intending to take.
The Bernina Express has also coaches that do not need reservation; these are mostly used by the locals. However, due to overtourism, this line is suffering from its own success. I would not take a train without seat reservation if you want to enjoy the journey. You may end up standing between the seats.
And, finally, seat reservations must be made with the line operator which is RhB, not with SBB.
Kind regards,
Roland