Yes, but the overall capacity is still limited by the 12 gauge wire at 20A.
The larger the gauge, the smaller the diameter. That makes 22 gauge wire not as thick as 18 gauge wire.
12 Gauge is thicker then the 20 gauge.
A tools that measured the size of a wire.
Yes. It is abbreviated as AWG. For example, in residential wiring the size wire to use on a 15 Amp circuit would be 14 gauge or 14 AWG.