Think of a Switch game file as a safe. Inside that safe is the game data (the code, the graphics, the music). However, the safe is locked. To open it, you need a specific key. Nintendo holds the "Master Key," but every individual console also possesses a set of unique keys derived from the manufacturer.
This is why official emulator projects like Ryujinx (which shut down after a settlement with Nintendo) and the now-defunct Yuzu included prod.keys with their downloads. They also could not provide links to get it. The emulator software itself is legal (reverse engineering for compatibility is often protected). But distributing decryption keys is not. prod.keys