• 国内こくない — domestic / within the country
• 案内あんない — guidance / showing around
• 内心ないしん — innermost thoughts
• 内科ないか — internal medicine
• 内側うちがわ — inner side

内 combines the radical for “enter” (入) with a cover or building. It originally referred to entering a space — like going into a house — and gradually came to mean “inside,” “within,” or “the inner part.”