different trades, worlds apart (idiom); to sb outside the profession, it is a closed book
Definitions (粵典–words.hk)
語句
另一個行業需要嘅技術同知識有太大分野，外行人唔能夠純熟掌握 different trades, worlds apart; to somebody outside the profession, it is a closed book; literally: another profession is like another mountain
Definitions (Wiktionary)
proverb
different trades are worlds apart; to someone outside the profession, it is a closed book