就 = 京 (a tall building on a hill) + 尤. It first meant 'to go up to / approach' a high place — to move toward and arrive at. From 'arriving right at it' came its grammar meanings: at once, then, precisely.
就 is one of the ten most common characters in Chinese, and a master of nuance. 'RIGHT AWAY / soon': 我就来 (I'm coming right now, with 来). 'THEN / in that case': 好，就这样 (OK, then it's settled). 'EXACTLY / none other than': 就是他! (It's him! / Exactly!, with 是). It also means 'as early/little as': 他八点就走了 (he left as early as 8). 就 is hard to translate word-for-word — you absorb it by feel — but it makes speech flow naturally, tying ideas together with a sense of 'right then, precisely so'.
Going right up to a tall building — arriving AT ONCE, precisely. THEN.
就 is a top-10 most-common character but hard to translate — 'just / then / right away / exactly' all at once. 就是! on its own means 'Exactly!' — strong agreement.