It’s a patch that requires a “legitimately” acquired ROM to work. This is how sites like SMW World avoid C&D’s, they host patches and mods and don’t have the actual ROMs on site. Nintendo can’t take it down because none of their code is there.
Play order for portable Zeldas: Link’s Awakening (DX is the same game with an additional dungeon), then both Oracles games (Seasons and Ages, there’s content you can only see if you play both), then Minish Cap, then Four Swords.
Get it now, before Nintendo finds out about it!