As I understand it, a plaintext encrypted using a One Time Pad is uncrackable because all possible plaintexts of the same length are possible. As a concrete example, if you decrypt the first 3 bytes of a 4-byte plaintext that you know is in English and get "HEL" (and you knew that was correct somehow), that does not help you decrypt the final byte. It is equally likely that the plaintext is "HELO", "HELP', "HELL", etc. and there is no way for you to know which is correct.

What is the concrete step(s) of AES (or a similar popular symmetric key encryption algorithm) that makes encrypting an n-byte plaintext with an n-byte key not the same as a One Time Pad?

It feels intuitive that if the key is shorter than the plaintext, then all plaintexts cannot be possible regardless of the algorithm. The other way around (key longer than the plaintext) is not intuitive.