Depending on how the game was dumped or downloaded, you will encounter different file types: : Raw disc images for the Wii U.
A ROM (Read-Only Memory) file is a type of file that contains data from a video game, in this case, BOTW. It's essentially a digital copy of the game's data, which can be used to play the game on a device other than the original Nintendo Switch console.
However, the golden rule persists: Not only does it violate Nintendo's rights, but it also exposes you to malicious files disguised as ROMs (keyloggers, miners). The only safe, ethical, and legal way to obtain a BOTW ROM file is to dump it from a console you own.