Could be a mix of northwest Indiana Chicago suburbs like Gary, as well as major college towns like Notre Dame and Lafayette, and maybe he did well in the Indianapolis suburbs too?
Obama's 2008 squeaker was not powered by suburbs. Obama ran well,
well behind Biden in suburban Hamilton County (61-38 McCain, 52-47 Trump.) Biden and Obama will get almost the exact same margin out of Marion County too (~63%).
Sure, Obama ran up his margins in NW Indiana (Lake, Porter, LaPorte, St. Joseph) but the real key was in strength in the rural parts of the states. For example, Benton County went from 70-28 Bush in 2004 to 57-41 McCain in 2008. In 2020, it was 73-24 Trump. Benton County is not in the Chicago media market either, FWIW
Obama's 2008 margins across the rural Midwest were insane (by contemporary standards, someone like Bayh, Edwards or
even Hillary Clinton in 2008 might have done better.) Obama won ~880 counties in 2008, and while this was actually a record-low for a winning candidate at the time, Democrats have managed to beat their own record again in 2012 and 2020.
