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?