If you were like me, then you probably thought those new files added in the latest snapshot didn't live up to the hype, but I finally figured out hot to do some basic stuff with them, and the possibilities are great. You can combine Amplified and Large Biomes worlds together or put those Floating Islands in the Nether!

CREATING THE FILE
1) Create a new world (Preferably a Default world). If you already have one, then skip to step 2.
2) Select that world.
3) Click "Edit".
4) Click "Export World Generation Settings"
5) The file, worldgen_settings_export.json, will be put in that world's folder.

If you open that file, then you will see the code is grouped by dimension.


TERRAIN
Each dimension has a line labeled "settings". This is the type of terrain that will be generated.
Ex: <"settings": "minecraft:overworld",>

To change the terrain, simply replace <overworld> with the desired terrain. Any terrain can be used in any dimension.
- overworld (default / surface buffet)
- floating_islands (1.13 – 1.15 islands buffet)
- end (end's default / 1.16 islands buffet)
- amplified (AMPLIFIED)
- caves (1.13 – 1.15 caves buffet)
- nether (nether's default / 1.16 caves buffet)


LARGE BIOMES
it is possible to use Large Biomes with any of these terrain types. However, I've only seen it work in the Overworld.
The overworld part of the code has the line <"large_biomes": false,>. Simply change <false> to <true>.


CHANGING THE BIOMES
Unfortunately, this part is trickier and appears to have fewer options. However, it is possible to do some useful things.

Each dimension had a group labeled "biome_source". This is what controls the biomes in that dimension.
It is possible to replace one dimension's "biome_source" with one from another dimension via copy and paste.
Ex. The nether's "biome_source" can replace the overworld's "biome_source" to create nether biomes in the overworld.

It is also possible to limit a dimension to a single biome via this code from a buffet world.
<"biome_source": {
"biome": "minecraft:plains",
"type": "minecraft:fixed"
},>

Simply replace <plains> with the desired biome's name. The biome does NOT need to be native to that dimension.


SUPERFLATS
I also believe it may be possible to turn each dimension into a Superflat, but the current snapshot has a terrible bug. It crashes whenever a player tries to create a superflat, including normal ones from built-in presets.

However, I was able to export the settings from a superflat created in an older version of the game, and it looks like that part of the code could be used in other dimensions. Unfortunately, we have to wait at least one week to see if this is true.


This is all I have discovered for now. Hopefully it's easy for everyone to understand. Also, my knowledge about this is limited, so it may be possible to do even more stuff with these files.