The modern fly inherited its genetics just as you inherited yours. The modern model is little changed because the original was a good "design," and the environment it fits has not significantly changed."Natural Laws and processes" are a result of a design. That's why information is trasmitted from ancestors to descendants since the beginning.
Who do you think was the first one on having the information that comes from the very beginning?
For example: if you compare a many millions years fly in amber and a modern fly, you'll see the same characteristics. That means modern fly received genetically the same information from its ancestor of millions of years ago.
It is a product of natural selection. Beneficial changes proliferate in the population. Individuals with non-beneficial traits tend to be out-competed, and their traits gradually eliminated.From where did that antiche fly received this information in the first place? It was created like that: designed, and ready to trasmit the same information for its descendants to receive it and transmit it on their part.
This process continues, for millions of years, gradually transforming simple microbes into myriad forms, of varying complexity.