Hi dodeqaa,

re:
> May I know how you reconstructed the curved surface please?

Well it's kind of tricky.. First used Edit > Separate on the 2 surfaces to break them out from the main object. Then selected the edges of the curved one and used Delete to untrim them and recover the full underlying surface. That then allows to select the side edge curve of it and use Ctrl+C/Ctrl+V to duplicate it as a curve and then delete the surface. Then positioned the lines from the bottom planar surface next to that side curve and edited control points to move the first interior point of the curved one so that it was aligned with the line's direction so they were smooth to each other. Then to fuse them together i used a trick where exporting a joined curve to SAT format and reimporting it will fuse tangent pieces together into longer curves. Then extruded that and trimmed it and joined it back in.

- Michael