Is visiting archaeological sites forbidden (haram)? And what is the ruling on benefiting from them? And if it is forbidden, why did the Prophet's Companions not demolish them during the conquest?

Visiting ruins varies depending on the intention and what results from it. If the intention is to observe and reflect on what became of those who came before—the great and the oppressors—then it is permissible, as long as it does not lead to an evil act or the neglect of an obligation, based on His (the Almighty's) saying: "Say, 'Travel through the land'" (Surat Al-An'am: from verse 11). However, if the visit is for the glorification of disbelievers and those astray, or for other corrupt purposes, then it is not permissible. The Companions' abandonment of ruins was due to people not glorifying them; had they glorified them, they would have demolished them just as they demolished idols, effaced statues, and leveled elevated graves.