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On the blockchain obviously, you interact with it with use of limit_order_create_operation/limit_order_create2_operation, but I'm pretty sure such answer would not be satisfactory :o)
It depends on what wallet you are using. When I need to use internal exchange I do it on https://wallet.hive.blog/market but I'm sure there are other tools that give you access to that exchange.

This was very helpful. I've been doing it inefficiently for a year. I appreciate the link. I bought and sold Hive and HBD without any problems at the link you provided.