How do we choose the probability for Russian roulette and at what luminance we should involve it at?

How is p_rr selected?

Is it true that in this case we don't bound the number of bounces?

Is p_rr more optimal when its value is large?

I am also confused what this means. If we didn't do this then wouldn't we have E(x/prr)? How does the expected value not change?

Wouldn't this increase the variance?

How do we decide what prr is