What does the intenal only toggle in the comunication tab do? Specifically, if the email was already sent to customer, and then the internal only wag toggled to Yes? I can see on the customer record that the email was logged

If you toggle internal Yes and choose your colleague as recipient the customer will not see that post in the message record. If you toggle the Yes after the email has been sent it will not make any difference as it must be toggled before send.