Minecraft wiki entry for Superflat_survival, associated text: Mushroom
To farm mushrooms, you need to bonemeal a mushroom to grow a huge mushroom. If you want to make a huge mushroom farm, you need to buy podzol from wandering traders. Mushroom can also grow on grass and dirt at low light level, but they cannot be exposed to the sky, so you can avoid buying podzol and create a roofed area to make it grow huge.