I don't get how they have the same races in a new galaxy, confusing?
That's actually one of the few things that has been hinted at in the survey leak:
Krogan colony ship has been stolen by outlaws. Track down the ship and return it to increase loyalty with Drack. Dialogue trees and end game impacted by loyalty status.
"Colony ship" pretty strongly hints at a galaxy hopping Ark of some sort ... as does the badge with A.R.K.C.O.N. on it (possibly some kind of convoluted acronym for Ark Contingency plan).
Personally, from what I've read, I'm going for some kind of Battlestar Gallactica, last hope for the Milky Way races, colony fleet type of thing - called the Ark because, screw it, we've had Space Jesus so we may as well have Space Noah ... with a sprinkling of Space Magic since the Black Ark theory won't have actually occurred to the writers.
Probably built and funded by some kind of inter-species secret NGO who, contrary to the Council believed Shepard's warnings, and launched at the time between ME2 and ME3.
Yeah, I'm expecting a lot of hand-waving by way of explanation.