There have been many attempts to find the historical Arthur, but these are, inevitably, unsatisfactory. There may have been several figures who put some wind behind the sail of this cycle of folklore, but even that is not necessary to explain Arthur and the many stories associated with him.

In reference to your question about whether the "myths were stolen" somehow, I respond by pointing out that folklore by its nature travels; the diffusion of stories is one of the hallmarks of oral tradition. There is no case to be made that it was a matter of literary theft, and so the term "stolen" should probably be set aside in favor of something less judgmental.

Several years ago, I answered something along the line of your question, and then I reused and shaped the answer to suit the many additional times I saw the subject pop up in /r/AskHistorians. Eventually, I integrated my answer to serve as an appendix in my Introduction to Folklore, which I have used when teaching the subject; I placed this answer together with other reddit-inspired replies. Here is the excerpt:

When I see the posts like this asking about whether there were real people or events behind legends, myths, and/or the ancient gods, I respond with several observations. First, the idea that the gods and heroes of legend are based on real people had an early proponent in the Greek, late-fourth-century BCE writer, Euhemerus, giving his name to this approach to myth and legend: Euhemerism. Folklorists generally regard the idea that there was an actual basis for most oral tradition as barking up the wrong tree, because the original “real” event behind a story is usually elusive and searching for that core is a futile exercise. In addition, research into how stories began usually concludes that they emerge in a rather spontaneous way, typically without an actual incident to inspire them.

A few examples: the Classical Greek story of Perseus is an early manifestation of a widespread folktale, catalogued by the twentieth-century folklorists Antti Aarne and Stith Thompson as AT 300, “The Dragon Slayer.” Was there a proto-Perseus who rescued a maiden from some sort of extraordinary threat or perhaps from some sort of human sacrifice? It is hard to answer that question, but it is not hard to imagine how far back in time that proto-incident would have had to occur: AT 300 is spread throughout Eurasia. It was collected from cultures that could have no conceivable literary connection with ancient Greece, and yet the shared assortment of motifs in the numerous variants clearly show some sort of genetic, that is, historically-connected relationship. Would we need, therefore, to go back thousands of years before the first recordation of the Perseus story to find this proto-Perseus? It is much easier to understand that the folktale simply diffused and that one of its manifestations was in ancient Greece.

Now, let’s consider another example that has inspired a lot of spilled ink. A simple Google search for the “origins of King Arthur” provides more websites than one could easily read in a week. Was there a proto-Arthur? Perhaps. Maybe there were several. But what does that prove? Every society has remarkable characters, and it may be a natural process for these sorts of individuals to attract all manner of traditional stories that have nothing to do with the original inspiration of the cycle of legends.

So, what do we have with Arthur? Was there a core source (or sources) for this legendary character? Let’s concede for the sake of argument that the answer is yes. Now, did this individual have a great warrior at his side who became ensnared by the leader’s wife in the fashion of Lancelot and Guinevere? That is more problematic since this type of story is also associated with Diarmuid and Grainne in the Irish court of King Finn and with the Cornish stories of Tristan and Isolde in the court of King Mark. One could even argue that it is the story behind Helen of Troy. In fact, it appears that this was a widespread type of story that became associated with various courts of historical legend. We cannot conclude that every great king had a queen who was attracted to one of his warriors and coerced him to take her away. This is simply a story that was attached to cycles involving great courts. In short, the further one goes back to find the “real Arthur,” the less the candidate (or candidates) look like the King Arthur who has been beloved for centuries. The proto Arthurs are not really King Arthur. They may be seeds but they look nothing like the tree that would grow over the centuries. We do not hold an acorn and say “Ah, I have in my hand a mighty oak tree.” It is not yet a tree. It is a seed. And the two look very different even if they are genetically linked.

One more example: there is a widespread legend told by countless families of the ghostly appearance of a loved one in anticipation of news that the individual died. This became a popular tradition in post-Famine Ireland because so many relatives lived in North America or elsewhere. But it is frequently told by all sorts of people internationally. So, we can ask, are there real-life, actual inspirations for this legend? That is, do the spirits of the dead actually come to visit loved ones? Well, how the hell should I know? To paraphrase a famous line from the television show “Star Trek,” “Damn it Jim, I’m a folklorist, not a ghost hunter.” And I have no intention of becoming a ghost hunter. It doesn’t matter what is behind stories so much as it does that people tell these stories. I’m in it for that part of the game; I consider stories as they are told over time, to gain from that material some insight into the past, into culture, and into the human condition. I am a folklorist. And with that, my plate is full.

Most of what we associate with Arthur was invented in the 15th century, by Thomas Mallory. However, Mallory was building off earlier works, most notably "The History of the Kings of Britain," written in the 12th century by Geoffrey of Monmouth. Lancelot originates in 12th century French material written by Chrétien de Troyes, and was incorporated by Mallory into the story we all know.

However, as the books I mention below lay out, there is some basis for the legend, and it predates Geoffrey substantially. Basically, there was a battle which we have reason to believe really happened, at Mount Badon (we don't know exactly where that was, although there are a couple of reasonable guesses), sometime in the 5th or possibly early 6th century.

Within a hundred years of that battle, the name "Arthur" (who is identified in a way that leaves it open to interpretation as to whether or not he was a king) was associated with the military leader of that battle. We have so few sources from that period that the absence of a more contemporaneous account isn't really evidence one way or the other.

When people talk about the "historical Arthur" they're usually talking about whomever it was who led the Britons in that (probably real) battle against the Angle-Saxon-Jutes, whether or not that person was named Arthur.

If you want a deep dive into the subject of the "historical" Arthur, as well as the geopolitical situation and an analysis of all the original sources, I highly recommend Mike Ashley's "A Brief History of King Arthur." He lays out all the primary sources, and draws his own conclusions, but does so in a way that acknowledges the room for alternative interpretations of the data as well as the general uncertainty about all things Dark Age Britain.

Christopher Gidlow's "The Reign of Arthur" goes over the same original sources, and, unsurprisingly, comes up with slightly different answers. I found Ashley's book to be a slightly more enjoyable read.

A third book, Geoffrey Ashe's "The Discovery of King Arthur" offers a completely different theory of the history Arthur. I found his case for a Breton figure ultimately unconvincing (Mike Ashley discusses it briefly in dismissing it) but thought his chapter on the geopolitical situation to be one of the clearest and best-written of any I read when I was doing a deep-dive into research in this area.

If you're doing more reading, start with Ashley. The other two books are a nice way to round out your knowledge on the subject if you want to go deeper.