To use a variac in electronics lab it is necessary to use one or more transformers...
- An 1:1 mains transformer, at the input...
- One or more transformers at the output of the variac, depending of the application(s)...
- Both the above, to be possible to have simultaneously an isolated mains and adjustable output(s)...
To decide which is the best solution depends on what the needs are...
Put down what you have, what you want to do and start studying/designing the solution. By searching here and there, vaguely without understanding, the result will not be the best...