Impedance is expressed in units of 'ohms'.

In a DC situation, impedance is simply the scalar resistance.

In a circuit where voltage/current have time-varying components,
impedance is a complex quantity, with both magnitude and angle.

I would hazard that the letter Z was chosen for because it's the last letter of the Roman alphabet. The connection is that Omega is the last letter of the Greek alphabet and is the unit symbol used for impedance.

Either that or its on heck of a coincidence!

Simple addition, but it must be done with complex numbers.

That depends on the output impedance.

In electronic we use voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times higher then the output impedance.

An input impedance is called also a load impedance or an external impedance.

An output impedance is called also a source impedance or an internal impedance.