Can a driver be assigned to all locations?

No, the driver needs to be assigned to a particular location. However, you can assign the driver to any other location at will.

To assign a driver to another location:

Step 1: Visit Setup > Staff.
Step 2: Search for the driver you want to assign to another location using the search field.
Step 3: Click the Edit button.
Step 4: From the Assign To dropdown, select the new location you want to assign the driver to. Click Update.