Welding a collection of edges only means that its resultant ‘curve’ will ‘extrude’ as a smooth object [no faceting] in subsequent operations.
If a collection of any edges is coplanar [welded or not], and it forms a ‘loop’ , then the way to add a face to it is to draw over one edge segment of it, and Sketchup will auto-face it for you.
However, if the edges do not form a closed loop, then no face is added.
Also, if the edges do loop but they are not all coplanar. then no effort will add one face.
That form then needs a ‘surface’ - i.e. a number of faces [probably triangulated] which can existing for some of those [coplanar] edges.
Triangulating will result in three connected edges which by definition is likely to produce a face [unless they are also colinear!]