Unlike the Protoss who teleport in, or the Zerg who hatch from larvae, Terrans come in space ships. Presumably, their command center contains many, many people living inside. But then you can construct a new building and it will magically be full of people (even if all your command center is flying far away on the map over a pit of lava).
Is there supposed to be a logical explanation for this? Or am I thinking too hard about it?
Lore wise, Terran units seem to be a mix of space humans and prisoners. Most of the collected lore comes from the books released (As the first StarCraft hinted that most Terran units where criminals but didn't explain it any further) but the first trailer for StarCraft 2 shows the process of a creating a Marine by the way of "releasing" prisoners and putting them into a new prison by the way of a power suit.
Since the release of SC2, the main site starts to reflect more about the units that are created being prisoners being forced to fight for early release of prison.
This does not explain all the units. Questions like, "Why are Medics and Ghosts built in the same Barracks with Prisoners" or "Are pilots professionals or prisoners" are not answered as well. For the most part though, we're lead to believe that every human is a separate/unique individual.*
*. Unique but of course it's all the same unit portrait in-game.
The question has already been answered, but I came across something of interest worth mentioning as a separate answer.
In the original Terran campaign on level 4: The Jacobs Installation, Jim Raynor and his team infiltrate a Terran facility, and at one point activate a "teleportation field" which moves them across the map. This is proof that the Terrans have access to teleportation technology, and would seem to explain (or at the very least provide a plausible explanation for) how it is possible for them to move units between buildings when required.
Through snooping around on an official StarCraft blog, it has also (quite accidentally) come to my attention that Terrans used cloning technology to populate the Koprulu sector (which could explain how they are able to rapidly produce units).
"This is an excellent question, and one that has been discussed internally for several years. I've always said that these were four crashed colony ships. So while much of their technology was lost, there were certainly means to ensure a foothold on a new, hostile planet – frozen embryos, frozen fertilized eggs, certainly extensive cloning."
This would also seem to be the only way to explain why a mind-controlled SCV is able to construct buildings that can be used to produce more humans.
Hey - if we can rapidly grow/reincarnate Zerg units (including intelligent ones like Kerrigan or the Cerebrate), why not Terrans?
