Unlike toy-grade vehicles that come with one pre-set frequency, with hobby-grade RCs you can switch out crystal sets of different frequencies to help reduce radio interference problems with other RCs in the area -- but the crystal for the receiver and controller of a single vehicle must use the same combination of band and channel.
Toy-grade RCs don't have receivers. The frequency is built in to the circuit board inside the RC. However, a crystal is soldered to the circuit board inside the transmitter of the vast majority of toy-grade RCs. So, although crystals control the frequency in toy-grade RCs, they are not removable. However, some toy RCs do come with a switch that allows you to switch among a pre-set group of channels which is not quite the same but similar to switching out crystal sets in hobby-grade RCs.