Because .nds files are often quite large, they are frequently shared online inside .7z archives to save space. However, most physical hardware like flashcarts and some emulators cannot read compressed files directly, requiring you to "convert" or extract them first.

Instead of converting, you need to the contents. This is a lossless process that takes 10 seconds.

.7z file → [Detect archive type] → [Validate contents] → [Extract NDS] → [Optional: Repair header] → .nds file