This is the passive of 펼치다 — nothing spreads it, it simply lies spread out or unfolds on its own. Most lines here are views, with size and openness adverbs: 시원하게 펼쳐진 한강 (the Han River stretching out wide), 넓게 펼쳐진 유적지, 광활하게 펼쳐지는. It also covers events unfolding — 어떤 일이 어떻게 펼쳐질지 모르기 때문에 (because there's no knowing how things will play out).