Cook County is such a huge place I'd love to see data at a smaller level. I'd guess that the north/northwestern parts account for the increase. Everybody we're seeing move into our area is coming from the south/southwest parts of Cook.
Gary continues to decline sharply and Hammond slightly, so Lake county overall is not growing quickly but certain parts certainly are.
Smaller level - these are the 9 most southwest/southern townships in Cook County, and their population change 2010-2020. I didn't get the official Census #s for 2020, these were the April 2020 estimates. The south suburbs ARE known for their high property taxes relative to the NW suburbs and City of Chicago. All but one are draining population...but if they all showed up on Lake County IN's doorstep, you'd think the county would show a lot more growth than half a percent.
Bloom - 90,977 to 87,470
Bremen - 110,069 to 106,197
Calumet - 20,770 to 19,843
Palos - 54,609 to 53,076
Rich - 76,656 to 74,752
Thornton - 169,378 to 162,029
Orland - 97,530 to 96,968
Lemont - 21,115 to 22,235
Worth - 152,607 to 146,491
Overall - 793,711 to 769,061 a drop of 24,650 (-3.1%)