How electric signal is converted to more powerful electricity
A power amplifier is usually what is used. Think of a microphone signal (just a few milli volts at very low current). The output from a microphone in electrical terms is barely a milli watt or so yet, this can be amplified up to incredible levels for blasting out over a tannoy, megaphone or the stage of a rock band (probably around 1000 watts).

Similarly power amplifiers can be used to control electrical motors from very small signals.

A "voltage converter" isn't really defined in terms of anything much - the real mechanism boosts power.