内部(nèi bù) - interior, inside; internal in Mandarin

内部 (nèi bù) is the inside of a structured thing: an organisation, a building, a machine, a party. It contrasts with 外部, the outside, and with 对外, facing outwards.

Most of its work is the English adjective internal. 内部消息 is inside information, 内部矛盾 is infighting, 内部价 is the staff price. Note that Chinese does not need 的 here; 内部 sits straight in front of the noun and modifies it.

The trap is using it for ordinary physical insides. The inside of a bag, a room or a box is 里面, and 包的内部 sounds like a technical drawing. Reach for 内部 when the thing has parts and a boundary worth talking about, and for 里面 when you just mean in there.