What does the "To" field mean?

The To field shows who the message is intended for. You might see something like firstmate@tidalmail.com or lighthousekeeper@harborpost.com. This field does not control delivery. Delivery happens through the invisible RCPT TO commands that are exchanged during the SMTP conversation.

For every recipient. Including CC and BCC. The sending server issues a separate RCPT TO command. The To line is only the human facing label that is written into the header once the server already knows where the message is going.

Think of it as the name written on the letter inside the envelope. The actual delivery address is on the envelope itself. The mail crew cares about the envelope. Not the decoration inside.