> Having genetic differences between males and females is mostly a bird and mammal thing, at least among vertebrates.

Also: the configuration and function of sex chromosomes is not consistent even within mammals. There are a number of species - primarily rodents - with unusual sex-determining systems, like species with XX/X0 (i.e. where males have an unpaired X chromosome) or even X0/X0.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8617835/