they will add a "salt" to the input as a means to prevent the user from being able to arbitrarily creating collisions.
Salt is added only to increase the difficulty of brute forcing if the database of that site was leaked.
The 'a' + 'b' = 10 formula is not a hash function. A hash function will always take a single input, and will produce the same output every time the same input is used.
I never said it is a hash function, it is one of many steps used in a hash function and if you can't reverse that, you can not reverse the hash itself!
This is the last thing that happens in SHA256 before the final hash is produced:
h0 := h0 + a
h1 := h1 + b
h2 := h2 + c
h3 := h3 + d
h4 := h4 + e
h5 := h5 + f
h6 := h6 + g
h7 := h7 + h