I'm sort of a beginner on cryptography so this is probably a really elementary question. I only know the historic ciphers and the general background on cryptography as well as it's general meaning. I'm just starting to dive into modern ciphers like RSA and DES. 


When looking at RSA, aside from using different keys for encryption and decryption, it seemed to me that the encryption process is sort of like a substitution cipher or better the shift or caesar's cipher (since this one does some kind of math at least), and that it operates on the characters' Unicode or ASCII instead.


Is there any way in which I'm correct or is there any way that the RSA could be sort of like a shift cipher that operates on the coded text instead of the normal one? I'm probably not correct so if so could you tell me the ways by which it makes it different or unique? Thanks in advance!