When a unit is morphed, what happens to it?
The unit's type value and stats change. The unit is technically the same. This includes Zerg Drones morphing into buildings(with the exception of the Zerg Extractor), units morphing into other units, and buildings morphing into other buildings. A new unit is not created from this.
What happens when Zerglings and Scourges are made?
What happens when a Command Center is infested?
The same thing that happens when a unit is morphed.
What happens when building over a Vespene Geyser?
The Vespene Geyser becomes (morphs into) the building. When a Zerg Extractor is placed, the Zerg Drone used to place it will be removed from the game. Cancelling the Extractor will create a new Drone which has full hit points.
What happens when I train a unit?
The unit is created, even though it is not visible or finished completing. Units that are queued are not created until it begins being trained and is updating the progress bar.
Where do units go when inside of bunkers/transports?
The unit is exactly where it was loaded. Even if you move a transport, the unit remains at the location it was loaded. It is merely hidden from view. Triggers will iterate through the transport's hold instead of detecting the unit directly. This also explains why moving Terran Bunkers in Use Map Settings will cause units to fire from where they were originally loaded.