This is the 2753rd most frequent Japanese word.
落ち着い
Calm; composed; settled.
In this sentence, 落ち着いた describes the atmosphere as calm and tranquil.
部屋の色調が暖かくて、とても落ち着いた雰囲気です。
The room's tone is warm and has a very calm atmosphere.
Here, 落ち着いている is used to describe someone’s state of being calm and composed.
彼は重要な試合の前でも落ち着いている。
Even before an important match, he remains calm.
In this context, 落ち着きました refers to the process of becoming calm or composed.