Radio Transmitter using Crystal Oscilliator
The voltage from pin 8 of SCTX2BC changes the DC component of the voltage on the base of Q2 transistor, modifying its quiescent IC current and its gain. So this is how the RF signal amplitude is modulated.

The RF signal from the Q1 based oscillator is connected via the capacitive voltage divider consisiting of C5 and C6.

The bias of Q1 may be found by the following equation:

$$V_{bias} = R7\,\frac{I_E}{\beta+1}+0.65V+I_E\times R8 = 9V$$

$$V_{CE} = 9V - I_E \times R8$$

$$I_C = \frac{\beta}{\beta + 1}I_E$$

C4 assures the positive feedback.