Phrase or idiom for only having seen one part of something but not the whole thing?

Solution 1:

Although I enjoyed learning the theory, I knew I hadn't seen the whole picture, because I lacked experimental courses.

the whole picture (idiom)

The overall sense or presentation of a situation, concept, topic, etc., including all related factors and potential consequences. You're focusing on one small part of the negotiation, but you need to see the whole picture—we may need to compromise on a few items to get the best deal in the end. Farlex Dictionary of Idioms

The function of university research was initially relevant for W. von Humboldt, because the assumption was that only someone who occupies themselves on a daily basis in verifying theories and discovering new truths can see the whole picture in a particular field... Aleksander Kobylarek; The Polish Humboldtian University in the Face of Paradigmatic Change