You might find this free tutorial from Mathworks (makers of Mathcad software) that covers the basics of brushless DC motors and how to control them.

https://www.mathworks.com/videos/brushless-dc-motors-introduction-1564728874059.html?s_tid=srchtitle

It is short and very basic, but provides a good starting point for understanding these machines.

I haven't searched their website, but they well might have other free learning materials dealing with various types of motors and their control.

so many nooks and crannies are part of this hobby,

Dennis