How to Make a Non-Planar Face (Remind Me!)

Nothing better than asking a question to a problem you’ve already solved - but forgotten how!

Below is the face of a curved driveway that extends downward that I want to replicate. I have the outlined geometry but I can’t seem to remember how I made this into a face.

Also, I was somehow able to push-pull it as well.

Faces in SketchUp are always planar; they are planar polygons. But the shape in your image does not look planar and your title says non-planar, so you are likely using the wrong terminology. A SketchUp surface is made up of multiple faces with their joining edges softened and smoothed. If you turn on View->Hidden Geometry you will see all the edges shared by the faces.

There are multiple ways one could create a surface such as you show, ranging drawing the curved edges and then hand stitching edges between their vertices to using any of various extensions such as curviloft. Since we weren’t watching over your shoulder, we can’t say which you actually used.

Wouldn’t “show hidden geometry” give a clue to stitching as a solution?

No, because the extensions also do the stitching, only automated.
Could be a different pattern though…

This must have been done by an extension like JointPushPull as push-pull only works on a single face. Not on a surface like yours…

If you have the 3D outline you can try if the Sandbox from Contours tool would give an usable surface.