What online courses should I take serially to learn WordPress?

I am a Web developer I use PHP/Javascript/HTML jQuery etc. I learn fast. I would like to use the ready-made components of WordPress combined with my programming skills to create things a bit faster. There are plenty of resources and tutorials online of course. Literally hundreds of courses in Lynda.com for Wordpress only. I know there are some core concepts I need to understand. Obviously all the courses would be repeating themselves to some extent. I am looking for the best way to get the core of it.

Like, how to program FOR Wordpress? Me, I am almost never happy with what's already made, and many times would like to make modifications. Do I need to learn how to develop plugins? can I simply modify existing plugins? For example if I want to use my own tables in the DB, are there any methods/plugins that make it more straightforward instead of writing my own code? Graphics. How to make my own graphics for the website? Hope you get me.

Thanks in advance!