Drop in a song you already own and MercuryPitch separates it into a vocal track and an instrumental one. Keep the instrumental as a backing track, or keep the vocal on its own to study how the line is actually sung. Then sing over it, with your pitch scored live against the original melody.
Open a file you already own and choose separation. MercuryPitch splits it into a vocal track and an instrumental one and keeps both, so you can mute either side. The separated stems are stored in your own browser, not on a server.
By default yes — it runs in your browser and the file never leaves your machine. There is also an optional server-side pass using a higher-quality model for the times a laptop cannot manage it, and that one is a deliberate choice you make per song, not the default.
Music you already own. MercuryPitch does not host, supply or download music, and there is no catalogue to search — you bring the file. What you do with the result is subject to the rights on the original recording.