Congressional districts need to be continuous , so that means populations density will get more seats. What they do with the extra population in the blue cities is they run the districts out to the burbs, ex-burbs and rural areas to capture as many red voters and bury them into a majority blue district
Yes US house seats Here is Chicago map of US house districts...... See how they take a block of Chicago blue voters then string the districts far into the suburbs to lock in as many red voters as they can but just enough to keep a blue majority This reduces the chance for more red districts When Chicago has an extra 100,000 illegals your strings can get longer and lock more red voters into the blue district Illinois is one of the worst states when it comes to gerrymandering