When should I use CC vs. BCC?

Use CC when transparency matters. You want everyone to see who is involved. You are calling the crew on deck.

Use BCC when privacy or discretion matters. You want to hide the list of recipients. Or you want to remove someone from a thread without making it awkward.

A professional move is to move the person who made the introduction to BCC in your first reply. They see that the connection happened. They do not have to receive the whole back and forth.