A variac is just what the name implies, a device
that, when plugged into a wall outlet, "varies" the
amount of "AC" line voltage available at the
"output" of the variac, hence the name VARIAC
A simple variac, the kind used in most ham shacks, is
nothing more than a large wire wound variable resistor
much like a rheostat.
A variac does not "produce" current. A variac's
current rating is the absolute "maximum" current that
can flow "through" the variac before the variac's fuse
will blow. The wire size and hardware used to
construct the variac determines the variac's fuse
rating, hence the variac's current rating.
A simple variac is not an isolation transformer.
A simple variac is not a transformer in the true
electronic sense of the word "transformer".
A simple variac does not "increase" the line
voltage the variac is plugged into. There are
some more sophisticated variacs that do but the
most common variac used in ham shacks don't.
When selecting a variac, the model chosen must have
a current rating that is higher than the highest
current rating of any equipment plugged into it.
Simple variacs provide no surge protection. Any surge
induced at the input to the variac will be seen at
the variac's output.
The ARRL handbook discusses how variac work. The
link below also provides similar information on