Making first mod, need some biome help.

I'm making my first mod, converting an old data pack of mine into a mod so that I might do some more advanced things with it. All of my new biomes, however, are Nether biomes, including new variants of the Warped and Crimson Forest biomes.

Issue is, I only see options for the default trees in the biome creator, and don't see various Nether-related features in the Features section of it, such as the Delta, glowstone, and fire patches. How do I add these to my biome?