How does a phreatic cave passage form?

Caves, and other karst features, are produced because limestone is soluble in water containing dissolved carbon dioxide and organic acids. Rain picks up atmospheric carbon dioxide as it falls. On passing through the soil more carbon dioxide, from plant roots and decaying vegetable matter becomes dissolved in the water, along with complex organic acids called humic acids. This ground water is easily able to dissolve limestone.

phreatic conditions result in three dimensional solution either forming large dome-shaped chambers or complex three dimensional mazes. The rock may become like a Swiss cheese producing what is called spongework.

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With respect, you've summarised cave formation but forgotten to define "phreatic"!

A phreati passage is one formed by dissolution as above but withinthe saturation zone of the rock mass' i.e. entirely filled withwater underhydrostatic pressure.Because the passage is totally filled the rock is attacked on all its exposed surface,so creating characteristically circular or elliptical passage sections.