ids = [int(row['id']) for row in b['cricket']['Arts & Entertainment']]
will give you a list of the numbers,
[6003760631113, 6003350271605, 6003106646578, 6003252371486, 6003370637135]
And for your new edits;
ids = [int(row['id']) for row in b['cricket']['Arts & Entertainment'] + b['News, Media & Publications']]
3
solved Parse a nested dictionary [closed]