True Random Number Generator by using avalanche breakdown
I checked the voltage from the red points in picture. But when I tested it, there was only a constant voltage of 8V. I thought it should be randomly changing between 8-0 volts. Is this normal ?

Yes that is normal. That part of the circuit (2 NPNs and the 15 kohm resistor) is a noise generator. It uses the randomness of the behavior of electrons in a reverse biased PN junction to generate noise.

That noise is then amplified by the amplifier which is the rest of the circuit (the inverters with DC feedback).

The noise is a very small signal so you cannot expect 0 to 8 V at the red dots. It will be in the order of millivolts AC. Since the NPNs are basically not conducting the resistor sets the DC voltage to 8 V.

This circuit is also very sensitive to disturbances from outside the circuit for example supply ripple like 50 or 60 Hz from the mains or switching noise from a switching DCDC converter.

So I would not expect a very "mathematically random" signal to come out of this circuit. Depending on what you need, it might suffice though.