Ok, So I believe it all started with Steemit where people used to post and the front end was exactly like the current Hive. Some jackass came and ruined it for all by some fraud. I am not very much aware of that fraudulent activity but I do know he did something like that.
Since these are all open-source platforms, Hive was built using the same source code, exactly a replica of Steemit. All the walltet funds from steem were transferred to hive.
POB is a community representing Hive. Everything that is posted on POB front end automatically lands up on Hive as well.