I don't see any numbers. Where did these millions of freed slaves go to leave the terrible suppression of southern slavery? How many were welcomed into northern homes and lands?
Part of it is called the Great Northern Migration. The rest had no need to really leave because of 2 reasons... they were no longer slaves, and the southern states had been destroyed and there was a lot of work that would be available during Reconstruction.
Are you Kanye West trying to argue slavery wasn't that bad because they didn't fight back hard enough?...
The 'Great Migration' wasn't really all that great', and didn't happen immediately after the Civil War, in any case. Northern labor recruiters were mainly looking for strike breakers and to fill the crappiest jobs in the meat packing plants. WW I was also a big draw. Auto plants, too.
There wan't that much 'work' after Reconstruction; it was still agriculturally based. Yankee troops didn't just loot white people's houses, they also robbed and murdered slaves; they couldn't believe slaves were allowed to own property, especially good stuff. Kanye West is right; most slaves were materially better off than 'free' white labor, north or south. Being 'freed' was no great favor bestowed on them by northern looters and killers.
In the South during Reconstruction there was a TON of work to do. So not only were there fields that were no longer manned by slaves, but a HUGE amount of the men who fought for the Confederacy in the war died or came back nothing like they left, many of whom would be missing limbs, eyes, or more. Those positions would be filled by former slaves, but eventually a second type of slavery would step in, where Blacks would be arrested and given fines they couldn't pay. A lot of the time being arrested for bogus crimes like vagrancy. White land owners would then come and buy their fines and make them have to work it off... which they never could because the land owners would charge more for food and shelter than they could afford.
So no, the Great Northern Migration wasn't at its peak right after the war, but it certainly started no long afterwards.