The hole in a real VW is a common sheet metal trick named "Rounded Louver". Realistic drawing of a louver needs complex shading. It's easiest if you can accept it as bent inwards, without shiny glosses and without a chrome edge list. Here are three of them on a flat surface:
One louver is made by interpolating between a black line and blue edge curve (=a line + two quarter circles joined)
The interpolation in Inkscape is Extensions > Modify Path > Interpolate.
The result can be a mess if the line and the edge curve have different directions. Flip the line horizontally if needed.
As you see the top edge isn't perfectly straight. Fix it by inserting a decorative list or simply a colored line which covers the edge.
I had 30 steps in the interpolation. Less can be enough if the line and the curve are wider. If you rasterize the louver shape the number of steps can be even higher without making the system slow.
Much simpler construction is possible if you can accept less realistic version which doesn't have rounded bottom curve :
The louvers are grey groups with blending mode Multiply. They make the paint surface darker.
The job starts again from an union of a rectangle and two quarter circles. Have 2 copies. The red version is used only as clipping path, it's color is meaningless. Fill the other with horizontal 4-stop grey gradient dark-light-light-dark:
Insert a blurred black rectangle on the top edge of the gradient shape. It will be the shadow:
Place the red version on the previous shapes, select all and apply Object > Clip > Set to get rid of the blurred extras. Make a group and give to it blending mode Multiply:
As you see the clipped shapes have a special "clip" symbol in the Layers panel. They can still be selected and adjusted freely in the layers panel. You very likely must adjust the blur of the shadow rectangle and the gradient when you have placed the louver onto the final surface.
Protruded louvers cannot be drawn by somehow reversing the colors. It doesn't look right because the major part of the louver should have same color as the surrounding surface. Here's a simple working version which can be used if the main paint color isn't black. It starts from the same basic forms as the previous version but it's colored differently:
1-2. place quarter circles at the ends of a rectangle and make Path > Union
Fill with car's main paint color
Change the fill to 4 stop horizontal gradient which has darker ends. Step 3 is useful because the stops can get easily the main paint color. Only remove the color transparency after you insert the stops by double-clicking. Then adjust the end stops darker
The louver cannot be seen properly on the final surface as is, it needs 2 additions:
- a thin stroke of the same color as the dark ends of the fill gradient (see NOTE1)
- a shadow
In the next image 4 louvers are placed on a flat surface. In the bottom you can see the shadow. It's actually the louver shape, but
- flipped and squeezed vertically
- with no stroke
- with black fill
- blurred and
- sent behind the louver
NOTE1 The stroke starts to look wrong if the image is so big that one sees the stroke is uniform. Blurring helps, but the stroke should be a separate curve if it's blurred. Harmfully there's no gradients along a path available. The stroke is not needed if the mid gradient stops of your louver are a little lighter than the main paint surface. But that creates a gloss. The same light condition should be used consistently in the whole car. It can fit because in your drawing example there's glosses caused by somehow downwards directed light. I tried to avoid glosses to keep things easily manageable.
