An apprenticeship is a training program where a young person is paired with a tradesman to learn the trade.
In Victorian England, apprenticeships were like indentured servitude. While the child was paired with the master, the apprentice belonged to that master. Usually someone had to pay the master. In this case, Miss Havisham pays for Pip to be apprenticed to Joe to be a blacksmith.
When I was old enough, I was to be apprenticed to Joe, and until I could assume that dignity I was not to be what Mrs. Joe called “Pompeyed,” or (as I render it) pampered. (ch 7)
Even though Joe is Pip’s uncle, and loves him very much, he still gets paid by Miss Havisham because that was the traditional way a young man entered an apprenticeship. Before receiving Magwitch’s “great expectations,” Pip was indeed learning how to be a blacksmith, but he stopped to get tutoring to be a “gentleman” in London from Matthew Pocket. This was more like tutoring or a school than an apprenticeship.