U.S. Census Bureau 2025 cartographic boundary files (20m): https://www2.census.gov/geo/tiger/GENZ2025/shp/cb_2025_us_state_20m.zip and cb_2025_us_county_20m.zip
Countries: Natural Earth public-domain 110m boundaries, via nvkelso/natural-earth-vector geojson/ne_110m_admin_0_countries.geojson. De facto boundaries; simplified for browsing. Insets are not geographically positioned.