If I have the encrypted RAR file, and I also have the unencrypted file, does there exist any way to find the cipher used by AES to decrypt it?
How does RAR encrypt files, separately, or does it encrypt them all in one? (in the case of unencrypted file name)
No you cannot find the key, that would require a brute force attack.
It encrypts the files separately, info that can be easily found on the internet.