Short answers, pulled from the story.
A unitary state is a sovereign state governed as a single entity in which the central government holds supreme authority. It may create, reshape, or abolish sub-national administrative divisions, and any powers held by regional or local governments exist only because the central government has chosen to delegate them.
The modern unitary state concept originated in France. The Hundred Years' War generated national feelings that unified France and accelerated its transformation from a feudal monarchy into a unitary state. France then spread the model through conquests during and after the Napoleonic Wars and through its colonial empire.
166 out of 193 United Nations member countries have a unitary system of government, making it the most common form of national governance worldwide.
In a unitary state, the central government is supreme and sub-national units hold only the powers the center delegates. In a federation, regional governments share power with the central government through a written constitution that requires both levels to consent to amendments, giving sub-national units guaranteed rights the center cannot unilaterally remove.
Yes. Scotland, Wales, and Northern Ireland hold devolved powers, but those powers are delegated by the Parliament of the United Kingdom, which may unilaterally alter or abolish devolution. This makes the UK a unitary state despite its significant regional autonomy.
Prefects serve as representatives of the French State in each department. Their function is to uphold central government policies at the local level, and they remain a visible illustration of the unitary model France originated and exported.