That is a good question, but probably a bit out of the subject matter for soil scientists.
But, I have had zoology classes, genetics, plant breeding, and am a dad, and in some ways the basics of the process are not much different from what happens in plants.
There is a process in the reproductive organs of both genders that produces cells with only half a set of chromosomes. When the male cells join the female cells, they have a full complement of chromosomes again, and a seed is formed in plants, and an embryo is formed in animals.
What happens next is cells in the embryo grow and divide and specialize into all the parts a baby needs to exist in the outside world.
This growth begins almost immediately in the mother’s womb in animals, but the seed must be planted in order to grow into a new plant.
Now, as for how the male and female chromosomes in animals get together, ask your mom or dad or some other adult in your family!