Flu is a bad example --- it has 8 "segments" (basically "chromosomes") that get reassorted among the offspring when a cell is infected by multiple strains, leading to a crude form of "sexual recombination". (The numbers after the "H" and "N" indicate the variant-numbers of the Hemagglutinin and Neuraminidase "segments" that the flu-variant got; a cell infected by H1N1 and H2N5 will also produce some H2N1 and H1N5 offspring in addition to H1N1 and H2N5, leading to very rapid genepool evolution)
By contrast SARS-CoV-2 only has a single chromosome, and so it primarily evolves via "genetic drift". While it's _possible_ for a cell infected by multiple SARS-CoV-2 variants to generate "chimeric" offspring via a process called "crossover" AKA "homologous recombination", that mechanism is much less effective than the simple segment-reassortment that drives flu evolution.
By contrast SARS-CoV-2 only has a single chromosome, and so it primarily evolves via "genetic drift". While it's _possible_ for a cell infected by multiple SARS-CoV-2 variants to generate "chimeric" offspring via a process called "crossover" AKA "homologous recombination", that mechanism is much less effective than the simple segment-reassortment that drives flu evolution.