If someone uses the spell command and says "drop", would you drop your backpack as well and all its contents, or just what's in your hands?
The spell is usually left up to the GM but "drop" has a specific effect
Much of the spell is left up to the GM's interpretation:
You might issue a command other than one described here. If you do so, the GM determines how the target behaves.
That said, the "Drop" command in particular is an example the spell provides:
Drop: The target drops whatever it is holding and then ends its turn.
So for this example a target would simply drop what it is holding and using the standard English meaning the target would not drop things it is carrying such as a backpack, let alone pour out all of its contents.