At this point you might ask yourself what does yellow filling is.
This is actually the autofill. And the autofill is what we will use to create surfaces or solids for.
So let me create another open spline. And then here this one endpoint I moved to there all these two lines overlap. And you see that it doesn't matter actually that the endpoints touch. All we need to do is just a sketch of any shape, where the ends at least touch each other or the lines overlap and then Shapr3D can figure out the interior fillet and we can extrude something out of it.