This part covers the basic theory of how transformers and autotransformers work.
An autotransformer only has a single winding, and although the voltage output is different from the input, there is no isolation between the input and output sides.
The same applies to a variac, which is an autotransformer with a continuosly variable output.
The second half of this video shows a practical demonstration of measuring voltages on the autotransformer, and how changing the position of the input voltage also changes the voltage on the output.