There’s plenty of debate over this as records are quite scarce, but the general current historical consensus appears to be that Arthur did not exist, but is an artistic representation of various British leaders.

For clarification on terminology - Briton or British at this time refers to the natives of the islands - those who would later become the Welsh. Wales derives from the Anglo-Saxon word for foreigner, whereas the Welsh word ‘Cymru’ derives from ‘fellow people’.

By the 500s the Anglo-Saxon settlement of Britain was in full swing, and there were frequent conflicts between the Britons and the Anglo-Saxon settlers. The Treachery of the Long Knives is normally attributed to the 5th century, which was when the Saxons turned against local Brittonic leaders (although this is somewhat pseudohistorical, like almost everything else around this period)

It’s quite hard to discuss this historically without discussing the literature around it. Arthurian legends only really began around the medieval period. I believe there’s a mention of Arthur in Y Gododdin, a really old Welsh poem. I think the first mention was in Geoffrey of Monmouth’s Historria Brittonum, but it really took off with Mallory’s Mort d’Arthur. This assigns medieval world views and ideologies onto a pseudohistorical character in a real historical setting. Most of what people “know” about Arthur comes from some sort of adaptation of Malory’s work, or of the later Victorian period revival spearheaded by Tennyson’s Idylls of the King.

As far as the connections to Wales, it’s important to remember many people mean ‘Welsh’ at this time to include Cornwall and Cumbria. Many Welsh legends do mention Arthur, and a lot of Arthurian myths come up in Welsh works like the Mabinogi. Carmarthen in South Wales is even named for Merlin (Caerfyrddin - Merlin’s fort)

To summarise - historical accuracy is nearly impossible when discussing this particular period of history, the line between myth and history is really blurry and almost all of the history tends to be very influenced by the myth. When discussing Arthur in particular, it’s important to realize he was probably not a real person, but a character within a wider mythology.