Maths has two different types of algorithms, linear and non linear.
Linear algorithms follow rules such as for addition:
As input x gets bigger and output y gets bigger, output z will get bigger.
The other type of algorithm is non linear for example taking the remainder after a decimal:
Regardless of the size of the input, we know the output will be between 0 or 1.
The sha algorithm uses blocks that are of the same size and runs a function on each pair of bits to return either a 0 or a 1 as an output for each bit. This is essentially the mathematical problem being solved.
(the problem with trying to fudge the numbers for non linear algorithms is that it's much harder to guess what second it is than what day it is)..
OMG, even reading your explanations on the matter, I still cannot understand it roughly

In short, is it the use of an algorithm and decoding what's inside it? Really confusing, today, most of us, we know BTC as a money-making tool, I must admit it