Anh,
Sketch a is the circuit diagram but Sketch b is the 'equivalent circuit'.
Note that in sketch b you show a capacitor Cb next to Rl but this should in fact be Co next to Rl. Cb shown in sketch a should be shown in Sketch b as being located in parallel with Vcc.
In a previous post, you asked about the use of the RFC (Radio Frequency Choke). This device was designed to be short circuit to DC power and open circuit to the signal frequencies. In sketch b, you show the power supply(Vcc), and I'm saying that Cb should be in parallel with the power supply Vcc. In sketch b, when the switch is closed, there is a current I, shown, flowing through Rsat. This current actually flows back to the power supply Vcc. The internal impedance of the power supply is generally not well defined for anything except DC current and it is good practice to ALWAYS shunt the power supply with an impedance which is open circuit for DC but short circuit for AC. Such a device which can do that, is a capacitor. That is the purpose of the capacitor Cb. As you will known from ohms law, a zero impedance will have zero volts across it, regardless of the magnitude of the current flowing through it. By use of the capacitor across Vcc, we can hold the signal voltage developed across Vcc at a zero value even thought he signal current could be large. This answers Q1.
Q2. The sketch b shows the resistance r sat. This is effectively the conduction impedance in the transistor. The value of rsat is dependent on the collector current AND the amount of base drive current. For pulse type applications (such as this), the base drive current is expressed as being either Ic/5 or Ic/10. If you look at device data sheets for switching transistors you will see what I mean. It is normally the case that in the engineering design phase, one looks closely at the power dissipation in the transistor. If the design under drives the base circuit, this can lead to excessive power dissipation in the collector circuit; and by increasing the power dissipation in the base circuit, this lead to lower total dissipation in the collector circuit. Thus, base drive arrangements are very important in establishing control on the collector dissipation.
Q3. The sketch is of a switching circuit. The switching frequency does not really matter from the point of view of understanding the DC and AC signal paths in the circuit. Such an arrangement as your sketch is used frequently at switching frequencies of 300 to 500 or 1000 kHz. A typical device could be a small welder, or a industrial eddy current heater for heating metal parts in an industrial process.
Hope this helps.