For example for a sentence-based password, 100 bits is insecure. But for a password generated using the secure random number generator, it is absolute unbreakable e.g.

No, that's not accurate. Bits of entropy are based on how many options you have for the type of password you used. The password "goodnight moon" has far less than 120 bits of entropy. A word-based password search algorithm would find it quite easily. Those are easily both in the top few thousand words in the English language, so that password probably has less than 24 bits (2*lg(4096)) of entropy.

12.5 ASCII characters

That assumes 8-bit ASCII. In practice, a password doesn't include characters 127-255 or 0-31, only 32-126, and probably not many symbols. One character selected randomly from characters 32-126 has ~6.6 bits of entropy. One character selected randomly from just letters and numbers, no symbols, has ~6 bits of entropy. One character selected randomly from just lowercase letters has ~4.7 bits of entropy. And if you don't choose randomly, and instead use a memorable password, then you reduce that entropy.

https://en.wikipedia.org/wiki/Entropy_%28information_theory%...