Like everything Arthurian there are disagreements. If Arthur existed he would have been a local warlord in SW England or Wales in the 5th or 6th century. He's missing from several of our earliest and best English sources. Bede for example makes no mention of anyone Arthur-like. So we have to wait at least three centuries for the Historia Brittonum before Arthur shows up. Of course most of the Arthurian tales don't take form for another couple centuries. Even the Historia only provides us with a list of Arthur's battles. Some of these, like Badon did actually happen, and have multiple sources. But most sources don't mention Arthur at all. So maybe there was an Arthur, but if there were, he was relatively insignificant in his own time.

Is Arthur based on a real person, or is he entirely mythical? We simply don't know. Given that, it's hard to parse out what else about the stories may be true. Much of the Arthurian stories are clearly mythological. And as his story became more popular it tended to accrue more myths to it.

That said some of it may have kernels of truth. An Arthur would have had a warband, men given land in exchange for military service. They wouldn't be "knights" as portrayed by later Arthurian authors. So there wouldn't be a chivalric code, nor would there be courtly love between knight and lady. But the idea of a bunch of unruly warriors, who squabble over rank and rating, and for whom the king's advisor commissions a round table so no one is closer or further from a head? That tidbit at least sounds plausible.

But plausible doesn't mean true. I like Thomas Charles-Edwards' summary from his The Arthur of History.

One can only say that there may well have been an historical Arthur; that the historian can as yet say nothing of value about him.

He then goes on for several hundred pages trying to expand that basic thought. But that sums up Arthurian scholarship pretty well I think.