Those arcs are most likely banding (also called posterization) in a smooth gradient like blue sky. It happens when subtle tonal transitions get reduced into larger steps, so instead of a continuous gradient you see visible bands or arcs where the tone “jumps.”

This is especially common in JPEG, because JPEG is typically 8-bit per channel and also uses compression that throws away subtle image information. RAW files usually preserve more tonal data, so the issue is often introduced during previewing, editing, or exporting to JPEG rather than by the sensor itself.

A histogram can reveal this: banding often produces a comb-like pattern instead of a smooth distribution.

To reduce it:

• edit from the RAW file when possible
• avoid heavy adjustments to smooth gradients
• export to higher-bit formats when available during editing
• add a tiny amount of noise/dither/grain if needed, which can make banding less visible

So your friend’s explanation isn’t quite right: JPEG isn’t “defaulting to white,” but limited bit depth/compression can make smooth color transitions break into visible bands.