- Just how did they capture a Collector Black Ark? That's the type of story that demands a 'little' bit of detail and action! Was there a great battle to secure this ship? Was it already carrying countless passengers at the time? Did the Leviathans, after being recruited by The Shepard, sever the ship's control from the Reapers and thus the Awakened pilots took those inside the ship somewhere safe? That would be something to see, indeed, and could help explain how the ship was captured.
Well, the forces of the galaxy involved with this project would have boarded and taken control of the vessel. With the information gathered both from James Vega and Commander Shepard our cycle now has immunity to the Seeker Swarms and an extensive enough understanding of their technology. So the only threat that remains is the ship's defenses and the Collectors aboard the vessel. With those surpassed and/or eliminated, the ship is ours.
- The ship obviously have stasis pods aboard, no need to retrofit (processing only occurs at bases, after all no worries about accidentally pressing the 'turn humans into kool-aid' button)? Its suggested that the Collector ship that harassed The Shepard could have carried a great many more people than it was simply by attacking colonies and that it would strike Earth at some point had The Shepard not intervened. In the year 2185, Earth had a population of roughly 11.6 Billion (including on the stations orbiting the planet). Now, clearly the Black Ark isn't going to waltz up to Earth and rescue everyone there, but the question of 'how much space does this thing have' is one to consider when you are bringing a large amount of races and no doubt a stockpile of supplies. . .then there's the whole dextro-levo thing.
The ship itself is hard to tell how much is fully dedicated to storage of stasis pods, but the chamber we enter is a few cubic kilometers and apparently can contain tens of thousands, hundreds of thousands, or maybe even millions of people since that chamber alone could contain every human colonist and still have room on just the wall, let alone the rest of the space. Plus they also have stasis pods scattered around in places other than the storage chambers. Add to that if we add onto the ship, the number of people it can support grows.
--Just how many people are brought total, and how is this divided up? Clearly the Reapers had an interest in humanity because We're Number One, thus we'd have the lion's share of seats, but The races they planned to use for Destroyer ships (Krogan, ETC) are sure to have a sizable portion too.
I can't really answer that. That's up to Bioware.
--How is it maintained? Having a Synthetic (Geth) and/or Pseudo-Organic (Awakened) tend to the ship's stasis pod-refugees is a far more cost-effective plan than a generational 'live ship' and who knows just how 'livable' a ship designed for utility and non-standard organic use would be. They'd need less resources to maintain themselves and would have 'some' knowledge on how to keep a ship going for a long period of time. . .not to mention explain how we are using the same general level of tech.
Most likely they would have either VIs, AIs, or Geth serve as the caretakers and pilots of the vessel while all organic personnel are in stasis. Plus then power can be diverted solely to the engines, controls, and stasis pods since synthetics don't need food, water, light, gravity, pressure, or air. Then when the ship nears the destination, those utilities are powered up so they are operational when the organic personnel awake.
---Sure the Quarians got by on normal ships, but this is a Black Ark! We don't even know what it looks like compared to a standard Collector Cruiser or if they're the same thing! Black Arks are only mentioned in ME3 and seem separate from the Cruiser ships. All this can be handwaved in comparison to 'how' the ship was taken, however, so its a relative non-issue.
The lore states that the only Collector ship on record is the Collector Cruisers, so the Black Arks are another term for them. We also know this since the Collectors in ME3 arrive in Black Arks and we see that those are Collector Cruisers in the trailer for that DLC.