Information on the boundaries data product and model is available at the National Map Boundaries website.
The USGS generally relies on data from external sources to support its requirement for boundaries as a core layer in The National Map. Some of the primary sources for these data include the U.S.
Boundaries are acquired from a variety of sources with varying resolutions. In general, boundaries cannot be used for legal purposes. Specific information on limitations of use is documented in the metadata available in the data download content.
Yes, you can extract data by some boundaries, including States, counties, congressional districts, and incorporated or unincorporated places.
You can order the good old-fashioned paper versions of the state by state indexes of USGS topographic maps online from 
Map service metadata is available in TNM Viewer by clicking on the pull-down arrow in front of the Governmental Unit Boundaries base data layer in the table of contents under Overlays.
Since boundaries are acquired from a variety of sources, the currentness of the data may vary by type of feature and by area of coverage.
The updates are variable. Since the USGS obtains most of this data from external sources, the frequency of updates depends on the release schedule of the source products.
Refer to the dataset attributes for the features. Download the data and view the feature attribute table to see these values.