You are correct that the popular interpretation of natural selection as "the strongest survive" is false. Evolution by natural selection is a much more nuanced process, and it involves more than just physical strength.
Natural selection is a mechanism of evolution that works by differential survival and reproduction of individuals within a population. It occurs when there is variation in a trait within a population, and that trait affects the ability of individuals to survive and reproduce. Individuals with traits that give them a reproductive advantage in their environment are more likely to pass those traits on to their offspring, while individuals with less advantageous traits are less likely to pass on their genes.
The traits that give individuals a reproductive advantage can vary depending on the environment. For example, in a predator-rich environment, traits that improve an individual's ability to detect or evade predators might be advantageous, while in an environment with abundant resources, traits that increase an individual's ability to acquire and store resources might be advantageous.
Physical strength is just one possible trait that could affect an individual's ability to survive and reproduce. Other traits, such as intelligence, disease resistance, and social skills, could also be important. So, while the strongest individuals in a population might have an advantage in certain situations, they might not necessarily be the ones that are most successful overall.
In summary, natural selection is a process of evolution that operates through differential survival and reproduction of individuals with advantageous traits in their environment. It is not simply a matter of "the strongest survive," but rather a more complex and nuanced process that depends on the specific traits that are advantageous in a given environment.