These previous threads answer your question pretty thoroughly:

How much of the legend of King Arthur is real? by /u/wtskm and /u/bitparity

Was there a historical King Arthur? And if 'yes', was his name Arthur or something else? also by /u/bitparity

Did King Arthur exist? by /u/alriclofgar

This thread on King Arthur origin theories is a good one too:

Historians and history fans, may I ask which theory for the origin of the King Arthur legend most appeals to you?

And if you don't feel like going through it all, this thread summarizes that one:

Where did we get the King Arthur lore from? by /u/Whoosier