It creates ciphertext using the alphabet like caesar cipher does.
Instead of moving the entire alphabet, it moves it with a case-by-case algorithm
Let’s take my name, izak and use this with our custom substitution cipher
A = 8
B = I
C = R
D = V
E = O
F = L
… (fillers, you get the idea)
I = X
K = ?
Z = д
It’d make Xд8?
I = X
Z = д
A = 8
K = ?
It’s a letter by letter replacement method.