Hello BogdanRO
For a couple of years, I have visited and worked in your country, in Iasi. To get from here to there, I took the tramway. To do so, I went to the next tram stop, I waited for the next service to arrive, I got my ticket, I boarded the tram, I had a ride, and got down at my destination. Some of the trams in Iasi operated in a Swiss city before, so were the buses.
So it works here with our trains: we go to the station, we check for the next departure (or check the SBB Mobile app), we get on the platform, the train arrives, we get on the train, we find ourselves a free seat, we have a ride, and we get off where we have to.
SBB does not know the number of passengers it will transport per train. But there are statistics from previous years, and the number of coaches is calculated based on the statistics.
In the rush hour, you might have to stand in the aisle or on the platform, but this more on suburban trains than on IC trains.
Kind regards.
Roland