Euler/Cardan Angles

I have a question regarding the explaination of Euler/Cardan Angles?
I do not really understand them, or really know how to apply this to
human movment. I know they are sequence driven and will change with
respect to the chosen sequence (i.e, x-y-z), but can not conceptualize
this for human movment. For example, when the knee bends, there is
motion in three planes...not specifically around one plane of
interest. I'll give an example...

Say I have a rigid body defined as my thigh (A), and a rigid body
defined as my shin (B) and I would like to describe the motion of B
relative to A during jumping (both A and B will be moving
simultaneously). This would be very similar to two rods attached by a
ball-and-socket joint. In theory we know that most motion will occur
in the sagittal pane, but will have some movement in the frontal and
transverse planes. How can we use Euler/Cardan Angles to determine
the amount of rotation occuring in the three planes, or about the
three axes?

Assuming the sequence is x-y-z. My understanding is that B would
first rotate about the x-axis of A, then around the "new, prime' " y-
axis of A, and finally around the "newest, double prime'' z-axis of
A". Is this correct?

How would this work if the initial two rigid bodies are not aligned/
coincident with each other?

How do you chose the appropriate sequence?

Is there a good text/website that can help me to visualize this?