How does offspring have a genetic variation

They have a variation because when two parents combine their genes, the offspring gets some from both and it continues every generation; getting traits and genes from all over the gene pool. Every person is different and has a unique set of genes, so when they are passed off, the offspring literally just has a bunch of everything

Through the process of meiosis. During meiosis, crossing-over occurs and increases genetic diversity. The haploid cells formed from meiosis (sperm or egg) have half the chromosomes so when the two meet they combine and share different chromosomes.