I guess you want the black area as a single open curve which contains straight and circular arc segments. No idea how to make it easily as a single open curve otherwise than by drawing it on a prepared set of guide or "helper" curves. In theory one could also splice and join together the actually wanted parts, but that would be a tough task.
Draw at first carefully with the grid the helper curve set - a collection of lines and circles which contain also the wanted route. I drew a few rectangles and circles to the 5x5 millimeter grid. The vertical midline was drawn as the last "helper" item with the Bezier tool.
All lengths an circle diameters fit to the grid because I had snap to grid ON among other snaps.
Group the helper curve set, lock it in the objects panel, turn snap to grid OFF and draw with the Bezier tool the wanted route. To get it perfect be sure you have snap to paths, nodes and crossings ON. Snap to grid causes troubles, because the curves do not cross the rectangles in grid crossings, so it must be OFF in my case.
I drew only one half, because I can make a mirror duplicate and join it:
As you see, my curved parts have one intermediate soft node dragged on the fly during the drawing. They are on the circle arcs, but otherwise they are extremely inaccurate. The fix is easy:
Path effect Spiro Spline rounds the arcs if they have corners at their ends and one symmetric soft node between them. All three nodes must be placed on a circle!
The fix become permanent by applying Path > Object to Path.
The full path was made by duplicating the drawn half, by applying horizontal flip and joining the halves at the end node:
The path is now ready. The helper curve set can be hidden or removed. The actual problem (the guessed one) is now solved, but some decorations may be wanted, maybe as extra items but without killing the "single open path" idea. Make for ex. the stroke wider:
Note: The result depends much on the used stroke style options.
A background shape will fill the white gaps. There are many ways to insert one. Here the path is duplicated and the original in the back got a wider green stroke: