1. Health and fitness
The Nazis were primarily concerned with exploiting the labor of Jews, so those who were healthy and fit for work were more likely to survive than those who were sick, disabled, or elderly. This is why younger Jews were more likely to be selected for labor, while older Jews were more likely to be killed.
2. Race
The Nazis believed that Jews were racially inferior, and this belief influenced their selection process. Jews who were considered to be more "Aryan" in appearance were more likely to be spared, while those who were more obviously Jewish were more likely to be killed. This is why Jews who could pass for non-Jews were more likely to survive, while those who were easily identifiable as Jews were more likely to be killed.
3. Religion
The Nazis also believed that Judaism was a threat to their ideology, and this influenced their selection process. Jews who were practicing Jews or who were known to have strong religious convictions were more likely to be killed, while Jews who had converted to Christianity or who had secular views were more likely to be spared. This is why many Jews tried to hide their religious beliefs, while others chose to openly practice Judaism even at the risk of their lives.
4. Luck
Finally, the selection process was often simply a matter of luck. Some Jews who were healthy, fit, and Aryan-looking were still killed, while others who were sick, disabled, or elderly managed to survive. This was due to the fact that the Nazis' selection process was often capricious and arbitrary, and there was no guarantee of survival even for those who met all of the criteria for survival.
It is important to remember that the Nazis' selection process was not about who was "better" or "worse." It was about who the Nazis deemed to be useful to their cause, and who they deemed to be a threat. The fact that some Jews were killed and others were not does not mean that those who were killed were in any way less valuable or less deserving of life. All Jews who were killed by the Nazis were victims of a senseless and unjust genocide, and their lives should never be forgotten.