[...] Carlos Slim, for example, keeps track of his huge business empire using handwritten notes, which he keeps in a series of meticulously cross-referenced journals.
I didn't understand this part: meticulously cross-referenced journals
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From your comment, you have trouble understanding 'cross-referenced' as it modifies 'journals'.
A journal that's cross-referenced is one where entries in the journal reference each other. Consider an encyclopedia - the article for 'volcano' probably references the articles for 'mountain', 'earthquake', 'lava', and 'plate tectonics', among other possibilities.
Similarly, the entries in Mr. Slim's journals reference each other. For example, a meeting he had in 2019 - about a new business venture in country X dealing with industry Y - might reference an unrelated vacation trip he took to country X in 2012, as well as all of his involvements in industry Y.
a notation or direction at one place (as in a book or filing system) to pertinent information at another place
verb definition 2:
to research, verify, or organize by means of cross-references
The quoted term 'cross-referenced' is the adjective form (from the past tense of the verb).