How was this accomplished? With some clever flavor text, of course.
Those few lines of italicized text below the rules text help to fill the believability gap when the name, mechanic, or art of a card don't tell the whole story.
Take the venerable Ornithopter for example. Why did this particular flying machine find its way to Mirrodin all the way from the time of Antiquities? The flavor text tells us:
"Regardless of the century, plane, or species, developing artificers never fail to invent the ornithopter."
A rip in time? Some sort of dimensional portal? No, it's just that the ornithopter is a basic engineering design that, given enough time and development of a civilization, always gets invented sooner or later.
What about the Brown Ouphe? It's not an invention. How did it show up on Mirrodin, all the way from the time of Ice Age?
"In a strange twist of fate, one of the most annoying creatures in the multiverse was brought to the place where it could cause the most damage."
It's not really a twist of fate, of course. Brown Ouphe the card has interesting interactions in a set full of artifacts. So it was included in the set for play reasons. But its flavor text reveals how those persons trying to build and maintain artifacts inside the setting might see its arrival as a fateful stroke. But why did it appear on the plane at all? You'll have to wait for Darksteel and Fifth Dawn to know the full story.
Yotian Soldier, like the Ornithopter, is an artifact creature whose design seemed to be unique to the Antiquities era. Yet it here it is, in a slightly chromier get-up, marching its tireless march around Mirrodin. What gives?
"Poets dream the verses of otherworldly stories. Artificers dream the blueprints of otherplanar artifacts."
So that's it. If Earth poets can be inspired to write verses from dreams, perhaps Mirrodin artificers can get inspiration from their dreams to design artifact creatures unknown on their home plane.
But how does that work, exactly? The ultimate answer is, of course: "Magic."