Topic category: Help with Minecraft modding (Java Edition)
I couldn't find a guide on how to make a proper cave biome for a custom dimension and after messing around for a bit I figured out how! So I figured I'd write it all down here to help other people.
Step 1 - Make Your Custom Biome:
This step is easy, make your biome like normal. Set all the generation parameters, blocks, features, structures, everything how you want. Make sure that "Generate Biome in Overworld", "Generate Biome in Overworld Caves", and "Generate Biome in Nether" are all UNTICKED (unless you want your biome generating in your custom dimension and these places).
Step 2 - Add Your Biome to the Dimension:
Another easy step. Like normal, go to your dimension element and add the biome.
Step 3 - Making Your Cave or Sky Biome Work:
Here it gets a little more complex, but still easy. Right-click your dimension element and select "Lock/Unlock Mod Element" and lock the code. Once you have done this right-click again and select "Open Mod Element in Code Editor". You will now see a bunch of files relating to your dimension. There will be multiple files all with the same name, being your_dimension.json, find the one located in the "dimension" folder and open it (you may also see files with this name located under "dimension_type" and "models/item", ignore these). Now that you have your file open you will see all the biomes you have created for your dimension along with the noise parameters you set. Find your cave/sky biome in this file. Under the "erosion" parameter you will find another parameter called "depth". By default this is 0, making the biome generate on the surface. Setting this to a value higher than 0 will put it underground. 0.5 is what most vanilla cave biomes use, 1 is used by the deep dark. Setting the "depth" value to a negative number will make the biome generate in the sky. Save the file and you should be good to go!
If I've missed anything or you have any questions please let me know, I will try my best to answer.