Customize World Generation Datapack

I have a World generation Datapack and want to Customize it however I have no clue where to start.
My Modifications arent supposed to be that hard tho all I want is:
-Villages to generate more frequently 
-Custom Structures that only generate underground generate over ground
-Have the Entire world just be two  biomes
-Change the world height, the Generated Underground needs to be much deeper than -64 or if thats not possibile the whole world needs to be lifted up to its Deeper