Vanilla Dimension Files

I am actually currently working on mod and want to see how my changes affect the biome source map. 
For some reason, the vanilla game does not seem to have the dimension files for the three dimensions in the data pack folder. Does anyone know A. why vanilla doesn't generate those dimesnions files naturally, and B. how does the game actually make changes when you put in your own dimension file. What exactly is it overwriting since I thought the dimension was hard coded.