The building designer is used to create design files which in turn appear in the game when you build something. Designs can be used for zone-related building or amenities.
Structures
To start creating a new buiding on the designer, you can choose the "New Design" option. From there, select a type of structure from the structure menu (hip, flat, gable,etc). Once you have select the structure you want to build,click and drag on the land grid to place the structure.
Once placed, you can change the structure's dimensions using the three handles. Use the top handle to control the number of floors and one of the horizontal axis. Use the lower handles to control the other axis and move the structure entirely.
Textures
When creating a building, you can go to the texture tab to change the texture that is applied to all placed structures. You can preview what texture would be applied to the current building based on its type (residential, commercial, office, government, industrial and more). Most textures are limited to one specific building type (ex: "residential" or "retail"), number of floors (maxfloors/minfloors) or heatmap value (minvalue/maxvalue). There are also textures that are limited to one specific design (ex: "design-police").
Decorations
You can use the decoration tab (third option in the lower left corner menu) to place non-structure objects in your scene. Click and hold to move them around and place them on the scene. After placing them you can use the handle to move them around. Decorations are colored using the scene palette and cannot use custom textures.
Saving
After you are done editting or creating a design you can save it and it will appear in the folder NewCity/modpacks/yours/designs where you can copy the files and share them with other game users.