Studios make money by offering recording services to solo musicians and bands. These services often include the initial recording session, mixing and later mastering of the recording.
Studios also make money by recording TV and radio commercials as well as soundtracks for TV shows and movies.
When a song is recorded, it needs to be mixed. Mixing a song involves adjusting the instrument and vocal levels, adding audio effects and sometimes rerecording one or more sections of the song.
Once the songs or commercials have been mixed they are then "mastered" which results in the final mix that can then be printed and sold.
Music studios will often hire an "in house" arranger and composer available to hire by private musicians and companies to write lyrics and music for their recordings or commercials.