Creating a key value pair, mapping a string to another key value pair.?

 d = {key: g.pivot(index='State', columns='County', values='FIPS').to_dict('records')[0] for key, g in df_test.groupby('State')}

d
###
{'California': {'Los Angeles County': '34027',
                'Orange County': '23421'},

 'New York': {'Mason County': '12345',
              'Stephan County': '12456',
              'Willams County': '54321'}

}

d['California']

###
{'Los Angeles County': '34027', 'Orange County': '23421'}

Apparently you want a third map made of keys from the second map, and matching values from the first map.
Map< String , String > thirdMap = new HashMap<>() ;
for ( Map.Entry< String , String > entry : secondMap.entrySet() ) {
    thirdMap.put(
        entry.getKey() ,                  // Second map’s key.
        firstMap.get( entry.getValue() )  // First map’s value.
    );
}