Here are a couple of ways to do it, there may be others that I can't think of at the moment.
First, you could go to cockpit view in a plane that you're flying and press 'Ctrl F1' until the cockpit and instruments disappear altogether, just leaving you moving through open sky.
The other way is to go to external view, on a plane you're flying or even an AI controlled plane, then press whatever button you have assigned to 'Toggle Independent Mode' in the Camera settings, then use the 'Hold To Zoom' and 'Hold To Offset' controls to zoom so far away from your plane that you can't see it anymore. This will leave you free to move through open space from more angles than the previous method.
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