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.
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.