If you are referring to the popular "Building Gadgets" mod (often part of larger quest packs), this topic usually covers the automation of complex structures.
Press the G key (default) to open the GUI. Here you can change modes (e.g., Build to Me, Vertical Column, Horizontal Wall). 18_its_just_a_gadget
Shift + Right-click a block in the world to set it as your "template" block. If you are referring to the popular "Building