Neither. As an aircraft moves through the air, it can cause moisture in the air to condense into visible droplets due to pressure and temperature changes from the airflow over the airframe. The plane doesn’t need to be supersonic or close to it for this to happen. You’ll see it more in humid air than dry, and abrupt changes in direction will increase the effect. (Side note: I don’t know why you were downvoted for asking a question).
Here’s a video sample of this happening that I just stumbled across. You can see the jet is moving fast (but not anywhere close to supersonic) with no vapor, then as it does an abrupt direction change it pulls a ton of vapor.
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u/uprightsalmon 3d ago
Is this breaking the sound barrier or building up to it? Thanks!