在 (zài) — (verb) to be (located) at; to exist; to be alive; (preposition) at; in; on (indicates location or time); (adverb) used before a verb to mark ongoing action

在 covers three jobs: locative verb ('be at/in'), preposition for time or place, and progressive marker before a verb (他∼吃饭 = 'is eating'). 正在 is a stronger, more explicit progressive marker; 存 and 存在 are more abstract 'to exist'.

Most characters — around four in five — are built this way: one part hints at the meaning, the other at the pronunciation.

The early form of 在 used 才 for its sound; a second sound part 士 was added later, and that 士 became the 土 seen in 在.