In our proposed home, there is a common toilet (powder room without bath) but it has no exterior wall. However, it shares a wall with another toilet. Can you please suggest a way to give ventilation/exhaust to the first toilet?
Having a vent on a powder is nice but not required. Since that room will not be producing high humidity the air technically doesn't require venting. If the home is already built, that is why it most likely doesn't already have one (track housing is almost always built just to code; if the code doesn't require it, it will not have it).
If the house is not built yet, just ask for a vent to be added to the plans. It can be vented to roof (if there is a roof directly overhead). It can also be vented to the side of the house, vent line will have to run in the cavity between floors.
If the house is already built, your options are limited. If there is attic over head, you can run a new vent easily. Otherwise you will have to wait to your next remodel to run a vent.