I switch between 2 modes in my gamecontroller. In one mode the player input is passed on to the player character (moving, jumping).
In the other mode I spawn in a gameobject which shows a preview of the current platform. This gameobject receives the player inputs in this mode, moving the platform preview, switching between different platforms. When the player presses the build button, I spawn an instance of the current selected platform at the place of the preview platform.
Good luck on your jam !