Experiments you can run yourself.
RSA security rests on the fact that factoring large numbers into primes is computationally very hard.
a mod n is the remainder of a division — just like the hours on a clock.
Shifting letters by k (mod 26); only 25 keys → easy to break.
Substitution preserves the frequency distribution — which is how the cipher is broken.
Changing a single input character completely changes the hash (the avalanche effect).