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How on earth did she pick up such a wealth of knowledge about fish?
Literal
She [topic-は] how fish [about-に関する] abundant knowledge [object-を] acquired [I wonder-のだろう].
The wondering tone hangs on のだろう (formal/written) at the end — explanatory の plus conjectural だろう, framing the sentence as a puzzled musing rather than a direct question. ~に関する is the formal way to attach 'concerning / regarding' to a following noun, and 豊富な knowledge ('abundant knowledge') is the kind of phrasing you'd find in a profile or essay rather than casual chat. 身につける (literally 'attach to one's body') is the standard idiom for acquiring a skill or body of knowledge through accumulated experience — distinct from 学ぶ ('study') in implying internalized mastery.