Code is layers from bottom to top separated by commas and biome after the semicolon. Layer thickness—the number with an asterisk in front of the block: 2*minecraft:dirt - two layers of earth, an unnumbered layer one block thick. Blocks and biomes - their id: they can be taken from ID block directory and on Biom pages.

It's not coded: the game takes them from the world's current settings. The error of the game does not show: a mistake in the name of any block -- and the world is silently becoming a classic plane, a typo in biome -- and the plains become biom.

You can assemble your code in layers - with block checking, Y heights and a line for server.properties - in flat world generator.