General Lore

Ben10Crew makes heavy use of a preexisting universe in which the show takes place, which is similar to the real universe, but different in several significant, often unsettling or ironic ways.

Timelines
One of the most jarring differences between our universe the show's is that the show's universe has demonstrated empirical proof of quantum mechanics through the involvement of diverging, interacting timelines. Beginning with the Timeline arc of season 3, the characters begin to find themselves in bizarre, nightmarish divergent timelines from which they have to escape. This also gives rise to evil dopplegangers running amok, which comes into play in later seasons.

The existence of timelines in this universe is never explicitly explained, so what we know about this is largely based on the speculation of the community.