In many different RPGs, magic is defined many ways, especially when it comes to creating or modifying parts of the physical world such as a wall of earth or a fog bank. In D&D 5e specifically, when creating magical darkness, magical fog, or even raising a small wall of dirt, what happens when the spell expires? Is it defined in the rules to instantly dispel all effects and return to its previously untouched state, or does it act naturally and slowly dispel the fog/darkness or cause the dirt wall to crumble and leave a small mound of dirt?
Many of the spells only explain how the spell is created:
You create a 20-foot-radius sphere of fog centered on a point within range. The sphere spreads around corners, and its area is heavily obscured. It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it.
I get that a wind can disperse a fog cloud, but what happens when the caster decides to end his concentration on the spell? Is this all flavor text left to the DM or are there rules related to this?