In the interest of education, I’ve been pondering how you might have created the model to get those errors in the first place and how you could avoid them in the future instead of having to fix it afterward.
The puzzle isn’t how come the thickness of the “C” and the “bar” don’t match. It’s easy to speculate why. Most likely, when you push/pulled the second one you either didn’t double-click to match the first push/pull, or you didn’t hover the cursor over an edge of the first object to tell SketchUp’s inference engine that you want to match it. A non-constrained push/pull got close but not quite.
What isn’t obvious is why the little face between the bottom edge of the bar and the top edge of the C is missing. Push/pull would normally have created that face. Did you delete it hoping to get rid of an “internal face” that was keeping SketchUp from considering the object to be solid? Or did you draw the object in some way other than push/pull?
Finally, because you push/pulled the two parts independently, SketchUp created a face to close the surface of the first one, and that face remains as an internal when you push/pull the second. To avoid this, you can draw the two shapes in the same plane, erase the edge between them, and then push/pull the resulting single face in one go: