World Stripper 1.15.1-1.14.4 modification will allow you to completely clear the selected part of the territory in which you are, while leaving only the underground generation. To do this, while in creative mode, press Delete (and to cancel you need to press Insert).

Screenshots

How to install World Stripper Mod

1. Download and install Fabric or Forge
2. Download mod
3. Unzip it and copy to C:\Users\User_Name\AppData\ Roaming\.minecraft\mods
4. Ready