The camp at Birkenau only had one gate, with an opening for a car and an opening for the train.
Upon disembarking from the cattle cars they'd traveled in onto the platform, the people were divided into two lines -- men, and women and children. Medical personnel gave a cursory glance to decide if you would be able to provide some days of work for the glory of the Reich, and further divided the people into those who were going to work, and those who were going to death. Most women went straight to death.
Identical twin children were saved, as Josef Mengele had joined the staff by this point and were using them for medical experimentation.
The Zyklon B step proved too effective for the end-to-end process, killing many people in a manner of minutes. Many more people than could be disposed of in the crematoriums. Therefore people who had been chosen for disposal often went into special "holding barracks" where they would wait for their turn in the Zyklon B chamber. With no food.