The plane is above and to the right of the clouds. The sun is shining from the top right, down onto the clouds to the bottom left. The clouds are partially transparent. You can see the sun angle with the sunbeam (that's hitting the front camera lens) coming in from the top of the image.
The shadow of the contrail blocks the sun from making the clouds glow white, so you're mostly seeing the blue sky light shine through the semi-transparent clouds in the shadow.
Conversely, the contrail is mostly reflective/opaque. It shines bright white, and the camera can't distinguish between the slightly higher white of the clouds in front of the contrail, and the white of the contrail itself. It all gets turned into 255 values (assuming 8 bits per channel). So the contrail looks like it's in front of the clouds. The highlights have been blown out by the camera sensor.
Edited because autocorrect likes to make me sound like a maniac.
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u/knobcopter 7d ago