What are artifact creatures in Magic lore?
Artifact creatures don't have consistent origins or properties.

The name "Artifact creature" represents the intersection of two fuzzy categories. "Creature" would seem to mean "animate being," but it also includes illusions, walls, and illusionary walls. "Artifact" usually refers to something that someone built, but it includes some natural phenomena like eggs and flowers.

A creature can have artificial material grafted into its body and become an artifact, like Breya, or Robocop. The Mirari possessed intelligence and a will of its own as a non-creature artifact; then it became/gave rise to Memnarch, an artifact creature (arguably "the same thing as the Mirari" but able to walk around).

I'm not sure why Karn should be treated as a special case: Prior to his tenure as a planeswalker, he had a long and illustrious career as an artifact creature: Built by wizards, without any organic components, but with his own intelligence, volition, and personality—essentially the same kind of entity as Bosh. (The fact that he's not an Artifact on his planeswalker cards probably has more to do with the designers' willingness to let him be destroyed by Naturalize, or the amount of space on the type line, than his nature in lore.)

The flavor text on Saheeli's Silverwing indicates that it's intelligent enough to understand spoken instructions. Akroan Horse is clearly inanimate, but still qualifies as a creature (in another decade it would have been a "Horse Wall")—but, still on the plane of Theros, Gold-Forged Sentinel is "beholden to neither" gods nor mortals, and flies around doing whatever it wants.

The "Construct" creature type evidently denoted artifact creatures that were not created magically—according to someone, at some point in time. This proposition does not track with the evidence. There doesn't seem to be anything magical about the Akroan Horse, but it's still a Horse and not a Construct. Creepy Doll, a Construct, was probably nonmagical when it was first built, but if it's not magical, then how does it stab things to death?

Artifact creatures can be intelligent or mindless. They can be magical or nonmagical. They can be animate or inanimate. They can be robots or cyborgs or neither. Their nature isn't consistent within a plane, and it isn't consistent within a creature type. There's no set of criteria that define artifact creatures, but rather a family resemblance that characterizes them in the minds of the artists, loreweavers, and designers. 

The nature of that family resemblance is hard to put into words without sounding dismissive, but I believe it really boils down to "An artifact creature is a creature that is, or at least seems, artificial."