How to understand this oscillating circuit made of two transistors and R C?
The principle of this oscillator is similar to other "relaxation oscillators" and oscillators that use a Unijunction Transistor, see this article.

The two transistors form a self-latching circuit, when the PNP conducts, it keeps the NPN in conductive state and that keeps the PNP in a conducting state. To get from the non-conducting state to the conducting (latched) state, the PNP transistor's base-emitter junction needs to enter breakdown. This generally happens at a voltage between 7 to 10 V. That breakdown will make current flow into the base of the NPN which will make it switch on. That then switches on the PNP as well. That will discharge C2, partly through the LED D2 which will light up.

When C2 has been discharged enough the current through the PNP and NPN drops so low that they will switch off again, releasing the latch. Then C2 can start charging again until the PNP enters breakdown and the cycle repeats.

Getting precise timing from this circuit is a matter of "trial and error", this isn't a very precise circuit as for example the breakdown voltage of the PNP is highly unpredictable, it varies even between the same model transistors. So you just build the circuit and adjust the values of R2 and C2 to get the timing you want.

If you want better predictability use a timer chip like the NE555 and even that isn't an accurate timer. To reach the next level of timing accuracy you will need a crystal as a time base in combination with a logic circuit or (much simpler) a microcontroller with a crystal as its timebase.