물건, 물체, and 물질 are a little bit difficult to understand at first when foreigners come across. I would say, 물체 is made out of 물질. 물건 is each item falling into the each category of 물체. 물체 can be a superordinate concept of 물건. Thus, 물체 is anything that has physical shape. ex) 쇠로 만들어진 모든 물체는 녹스는 단점이 있다. 물건 is a specific item, so we usually use this word in daily lives. ex) 물건을 부수면 보상해야 한다. 이 상품권은 물건을 구입할 때 쓸 수 있습니다. 물체 can be used something without detailed information, so it is commonly used for UFO, or things that's not yet revealed in the news. ex) 미확인 물체가 발견되어 확인 중입니다. c.f. There are always exceptional meanings in words. 물질 can refer to money or possessions. ex) 물질만 쫓으면 지금의 행복을 놓치게 된다. You can say 물건 for person when you think they have some talent or unique thing and it can be used in a positive or negative way depending on the context. ex) 이번에 신입사원 중에 물건이 많아.

Your understanding is correct. * 물체 = a physical object. Narrowly defined formal or technical term. (물 = physical thing, 체 = body) * 물건 = a thing, object, item, merchandise for sale. (건 = a unit item). Widely used everyday word. It also has some slangy meanings, like when referring to a people as a character. Examples: - 미확인 비행 물체 = an unidentified flying object. - 어둠 속에서 다가오는 물체가 희미하게 보였다 = An approaching object was faintly visible in the dark. - 남의 물건은 손대지 마세요 = Do not touch other people's things. - 저 가게는 좋은 물건을 싸게 판다 = That store sells good stuff at low price. - 그 친구는 참 대단한 물건이야 = He's really quite a character.