Translation guide
The English word 'internal' can refer to things inside a body, organization, country, or system. Japanese uses different words depending on what is 'internal' and the context.
Specifically refers to internal organs (visceral). More anatomical than 内部の.
内臓脂肪
visceral fat
Referring to matters, communications, or structures inside a company, government, or group.
Referring to affairs, politics, or economics inside a nation.
Referring to thoughts, feelings, or qualities that are inside a person.
Referring to components, processes, or errors inside a device, software, or system.
内部 (ないぶ) is more formal and often used for abstract or organizational internals. 内側 (うちがわ) means the physical inside or inner side of an object (e.g., the inside of a box). For 'internal' in the sense of 'inside a container', 内側 is more natural.
箱の内側
the inside of the box
English 'internal' is often translated as 内部, but in many set phrases, Japanese uses specific compounds (e.g., 国内 for domestic, 社内 for company-internal). Using 内部 for everything can sound unnatural.