“Not only/but (also)” is one of the correlative coordinate conjunctions. It connects grammatically equal parts. The word “also” is often left out or moved to a later position
in the sentence, and it doesn’t make any difference whether it is there or not.
When this combination links two sentences, there is a comma before the “but.” When it
links anything dependent, there is no comma.
…Not only did I see him, but I had a chance to chat with him also.
…She not only doesn’t know that, but no one is willing to tell her.
…She was not only angry but also hurt.
…I visited not only Guadalajara but also Mexico City.