在于 zàiyú to rest with

to rest with; to lie in; to be due to (a certain attribute); (of a matter) to be determined by; to be up to (somebody)

How to remember 在于: 在 at + 于 at = to rest with.

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在于 (zàiyú) is a formal linking verb meaning "to lie in / to consist in," used to state where the key point of something rests.
Classic pattern: A在于B: 成功在于坚持 (chénggōng zàiyú jiānchí), success lies in persistence.
It's written/argumentative register, essays, speeches, explanations of cause, not something you'd casually say about small talk.
Similar to 取决于 (qǔjuéyú, to depend on), but 在于 pinpoints "the essential factor is," while 取决于 frames an outcome as contingent on a variable.