If my song is approved, does that mean that it will be on a compilation?

If your song is approved, that means that the song is available to be compiled on compilation albums.

From that point on your song is available for connected curators (A&Rs) to preview and to use on compilation albums. Approval doesn't mean your song is already compiled.

Anyway, you have the option to buy guaranteed placements on compilation albums, once your song has been approved.