The appendages of cockroaches and turtles are modified for creeping movements, but their internal structures are completely different from each others. what could be the possible interpretation for this similarity

A convergent evolution is a phenomenon of independent evolution of similar traits in species that are in different lineages. These traits are called analogues structures. They are similar in form or function but were not present in the last common ancestor of those species. So, the appendages of cockroaches and turtles are analogues structures with the similar function - creeping movements, and they are the result of the convergent evolution, but they evolved from two different ancestors.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).