Your orange VW example is coarse. The groove between 2 surfaces is only a curve with darker color. It can be grey with blending mode multiply. The darkness of the grey is different in different places. Partially transparent black can also be used. It works with normal blending.
I guess you expect something which stands bigger size. Dark curve is plausible only if the curve is shown about 1 px wide on a display.
The next example is a little more complex. There's a dark gap between the red surfaces. The edges of the surfaces have strokes which are colored to make a little glossy, but coarse shading for a light which comes from the sky. The gap is black stroke.
It's three adjacent curves drawn on a flat surface, black in the middle and around it there's strokes with vertical grey gradients. One is dark to light and the other is opposite light to dark.
The gradient strokes have blending mode Hard light, so the over 50% grey appears as highlight and the under 50% appears as shadow.
I see this is a little lousy. Curves should be splitted to smaller pieces which should have different gradients. This is again an example how glosses make things complex. Besides three exactly adjacent curves are tricky to draw (see NOTE1)
If you can live without glosses you get plausible grooves with rounded edges very easily:
It's simply 2 copies of a black curve on a solid flat paint surface. The bottom curve is blurred to make an impression of curved edges. The top curve is solid. You can reduce its opacity or change its color lighter if deep black feels too deep. In the example the opacity is 75%.
Blending mode Normal works well because there's only partially transparent black, nothing is made brighter and there's used no white.
Here's the same curve zoomed out:
You can insert glosses here and there as separate light shapes but it's not beginner's job. Making it well takes the direction of the light into the account consistently. Here's an example how the glosses could be placed. The groove is wider to make everything well visible and the light comes from 3 different directions behind the watcher:
Do not assume I had patience to draw these glosses. They are made automatically by using an entry level 3D CAD program (=DesignSpark Mechanical, the free version). In 3D you get different views from the same drawing with no effort. Here's one:
NOTE1: Three adjacent curves are tricky. Here's one method for closed paths. Other methods exist for ex. based on "Offset paths" or on "Pattern along a path". This is exact and simple and the curves stay single stroke paths.But it works only with closed curves. You can insert a dummy part to close an open curve and split it off later if you need adjacent open curves.
Draw a curve. Here's one. The stroke width = 0,5 mm. Make a duplicate. Ctrl+D makes one on the top. Let it have double stroke width, in this case 1 mm.
In the next image the duplicate is moved temporarily apart and recolored to show it.
Beware moving parts! Later they are impossible to get aligned back. Snaps will not help later!
Convert the duplicate to path with Path > Stroke to Path. It becomes filled area. Remove the fill color and insert 0,5 mm stroke:
The magenta part is now Combined Path which occurs as one item in the Objects panel.
Split the combined path to 2 separate paths. Apply Path > Break apart. Then you can select and recolor the strokes independently:
All three curves are now independent single stroke paths with no fill. They can be for ex. splitted with the node tool to make complex colorings possible.
As said, beware moving the curves apart. They are virtually impossible to get back aligned. Making a group helps, but the parts can still be easily selected individually by double clicking or via the Objects panel.
