the author does not get deep into when/what to put into the 'second brain'.

Just like one does not go to the market unless it is not in the pantry, one should not replicate what google can get for you in a second.

So, then, what needs to be in the second brain is more of keywords and categories, so that we can make the google searches more effective for the particular question we have

P.S.: assuming we will always have google/some search engine.