Do you mean "how do I duplicate this artwork"?
Lots of ways. Try overlapping two circles and drawing a dissecting line. Convert all to a "Live Paint" object and set fill to "none" in the negative areas. Select the arch where the circles overlap, with the Live Paint Selections tool and set it's stroke to white.
If I see what you see (a reddish circle with a segment folded in over the rest visible by its white curved border), you may, with Smart Guide on:
1) Create the reddiush circle with a white stroke,
2) Create a path with the Line Tool or the Pen Tool to cross 1) where it is to be cut,
3) Object>Path>Divide Objects Below,
4) Select the segment and Transform>Rotate by 180 degrees, move it to the front if needed,
5) Drag the segment (or the rest of the circle) by one of the relevant Anchor Points (where it was cut) until Smart Guides say Anchor Point, and you are there.
1. Create a circle with fill color.
2. With Scissors Tool cut the circle's path where the endpoints of the straight edge will be.
3. With the Selection tool select the smaller of the two arcs formed by cutting the circle.
4. With the Reflect Tool click the two endpoints of that arc.
5. With the reflected arc still selected, apply white stroke.
Here are some visuals to go along with Jacob and Peter's great instructions, just in case they help.
1) Create a circle with fill color.
2) Create a path with the line tool where you want the flat side.
3) Object > Path > Divide creates two separate objects.
4) Select the smaller segment and Transform > Rotate 180 degrees and reposition if needed.
5) Apply a white stroke to the smaller segment.
Thank you Jacob and Peter! And good luck SebastiaoV!