A few questions about ciphers?

Question

Algorithms

Hello, I'm doing a little bit of cryptography, can you please tell me in such matters?:
1) The shift cipher is an absolutely stable scheme, since it uses a one-time key, and the key size is the same as the message size. Can a shift cipher be considered absolutely stable if it does not use modular arithmetic operations?
2) Another interesting question is why simple substitution or permutation ciphers, despite their vulnerability to frequency analysis, are still widely used in modern encryption algorithms and cryptographic protocols?
3) Assume that in the text cipher block chaining (CBC) mode, the result of decryption is "correctly padded text". Can it be argued that data integrity is preserved in the transferred source text?
Please, the answers to these questions are important to me

Answer 1

do some cryptography

And what actions in the study/development of the algorithm/etc led you to such questions?!