The Duchy of Cornwall was created out of the former Earldom of Cornwall by Edward III in 1337 in order to provide an income for his heir, seven year old Edward of Woodstock (later known as the Black Prince). From that day to this, its purpose has remained the same.
An earl is a member of the nobility. The title is Anglo-Saxon, akin to the Scandinavian form jarl, and meant "chieftain", particularly a chieftain set to rule a territory in a king's stead. In Scandinavia, it became obsolete in the Middle Ages and was replaced with duke (hertig/hertug). In later medieval Britain, it became the equivalent of the continental count (in England in the earlier period, it was more akin to duke)
The Normans and Anglo Normans created by means of a cultural appropriation
of Cornish history a ruling elite that holds power to this day. There is a King Donyarth but by the 9th century things were very bleak for what formerly would have been extensive territories and the claimed 'lineage' is to bolster elite rule.
The term Duke originates from 'Dux' and serves a specific function;
After Diocletian's Tetrarchy reform, the provinces were organized into dioceses each administered by a vicarius. As with the governors, the vicarius was assisted by a dux. This dux was superior to all of other duces within the dioceses and when the vicarius called the legions of the dioceses into action, all of the legions were at the dux's command. An example would be the Dux per Gallia who was the dux of the dioceses of Gaul. The office of dux was, in turn, made subject to the magister militum of his respective praetorian prefecture, and above him to the emperor.
The 'Earls of Cornwall' were Norman or English or French (even some from Spanish decent) overlords and their function from 1066 was to collecting revenue for the crown - by whatever means.
John of Eltham, 1st Earl of Cornwall (15 August 1316 – 13 September 1336) was the second son of king Edward II of England and his queen Isabella of France. He was heir to the English throne from the date of the abdication of his father (25 January 1327) to the birth of his nephew Edward of Woodstock (15 June 1330).
so therefore the title after 1337 moves to the eldest son (ie; the to-be monarch) rather than their vassals - though the vassals were always answerable to the monarch.
Today of course they shore up the same power structure with various titles to the same means such as the Lord Lieutenant and the High Sheriff - each to feebly give the impression of devolved power but in reality ensuring continuity;
The Office of High Sheriff is an independent non-political Royal appointment for a single year. The origins of the Office date back to Saxon times, when the ‘Shire Reeve’ was responsible to the king for the maintenance of law and order within the shire, or county, and for the collection and return of taxes due to the Crown. Today, there are 55 High Sheriffs serving the counties of England and Wales each year.
Of course being appointed by the crown they are neither non political nor independent.
Anyone who says the constitutional basis for the administration of Cornwall as part of England, arguing that the Duchy Charters of 1337 place the governance of Cornwall with the Duchy of Cornwall rather than the English Crown' is confused and mistaken because the Duchy of Cornwall always was
either the Norman or the English read 'Anglo Norman' crown - (originating from and perpetuating foreign rule) and therefore has no legitimate jurisdiction.