The Founder Effect refers to the relationship of the genetic diversity of a population to the limited genetic diversity of the founders of the population. Chances are that those individuals represent only a small portion of the genetic diversity in the larger cat population. This is similar to the concept of bottleneck, but we are not dealing with the death of a majority of the original population, just looking at the effects of migration of a small group that establishes a new population. Genetic drift refers to changes in allele frequency resulting form random factors that affect which cats survive and reproduce. It is not a form of natural selection.
Some cats are just lucky and produce more offspring than others.
Some individuals may be killed by natural disasters or accidents that are not related to their phenotype or genotype. As a result, and especially in a small population, genetic drift can lead to drastic shifts in gene frequency simply because individuals carrying a particular allele do not survive or fail to reproduce. Again, this is NOT selection based on a particular phenotype, just luck. I am describing this example as an example of gentic drift that would not be classified as a bottleneck, founder effect, or gene flow. There is no establishment of a new population, no drastic reduction in population size, and no migration.