the independent variable is the array tt, which appears only once in the assignment to the variable zz, here it is contained in the complex exponential. mathematically, exp( ix ) = cox(x) + i*sin(x), this is Eulers formula, and hence, the real part of zz will come from the cosine function, and this is why it is oscillatory.
Amplitude, you should know. For calculating phase shift, you're looking for the horizontal displacement between the two waveforms. in other words, cos( x + m ) for variable x and constant m, is said to have phase shift m. You can see the phase of this waveform hiding in your expression for zz, in the form of the constant exponential factor. Try using the algebraic laws of exponentials to make this expression look like cos( x+ m).
