Why is it "in there" instead of "in that"? I thought the object they're talking about is definite so "that" sounds natural.

Native speaker’s answer
Rebecca
"In there" is used here rather than "in that" because the object in this scene is hollow on the inside, making it an enclosed space. Because it is an enclosed space, it is considered a "location". Therefore, "there" is used because "there" refers to a location. "Get something in there" is also a standard phrase native English speakers use. Ex: Will you help me put my jacket in my backpack? I can't get it in there.