Yes, they're decided by population
There's no real 'set population'... politicans pretty much try to keep them as even as possible! Well, hopefully they do
Counties really don't factor into Congressional District creation. Take into consideration some of Illinois's CDs:
A lot of which are in one county (Cook).
Illinois also gives an example of gerrymandering. Gerrmandering is purposefully making a district look 'weird' to give political advantage to one party or another. It's really confusing sometimes
