When it comes to learning/understanding Complex Numbers there is one pre-eminent source which everybody should lookup;

A series of videos named "Imaginary Numbers are Real" by Welch Labs - https://www.youtube.com/playlist?list=PLiaHhY2iBX9g6KIvZ_703...

You can buy book versions (used to be free earlier) at - https://www.welchlabs.com/resources