You are correct that frequencies that high would be completely unmanagable. Sending one bit per frequency would cause problems for various types of radio transmissions as well. So we have modulation techniques which allow more than one bit to be send.
A touch of terminology: baud, most people will remember that term from the days of telephone modems, is the symbol rate at which a communications medium is operating. A symbol can contain more than one bit however.
1 GbE uses a symbol rate of 125 MHz (125 Mbaud), uses all 4 pairs for both directions at all time, and uses a trellis coding to enable 2 bits of data to be sent in each symbol (along with some error detecting information). Thus 125M/s * 4 * 2b = 1Gbps.
100MbE used the same symbol rate of 125MHz, which is why only a minor improvement was needed for the existing Cat 5 standard to carry 1Gb signals, specified as Cat5e
10GbE uses 500Mhz, 4 pairs, and codes 5 data bits into each symbol; 500M/s * 4 * 5 = 10Gbps. 40GbE and 100GbE have no copper standards at this time (there is a rudimentary draft that uses 1000 Mbaud, 4 pair, with 10 data bits and 25 data bits codings, respectively). There is also a proprietary "backplane" specification and non-standardized fiber specifications.