Be careful with "Free Download" sites for game ROMs, as they are often packed with malware. It’s always safer to rip the files from your own physical copy if possible.
You'll need the latest version of Yuzu (or a fork like Suyu, as Yuzu development officially ceased in 2024). Super Mario 3D All-Stars Free Download With Yuz...
To run the game legally and smoothly, you need the prod.keys and title.keys from your own Nintendo Switch, along with the game's .xci or .nsp file. Be careful with "Free Download" sites for game