Morricone writes the music. -Creates it. Usually at a piano. Sometimes they're just bits-of-tunes that form in his head while he's going about his day doing other stuff.
For a specific film, he'd usually get tapes of director-choice scenes as the film is being shot. Then watch them while sitting at the piano. Some directors give him 'hints' about what the music should be highlighting. A musical theme develops slowly. Main characters will probably inspire their own theme bits, played on character-specific instruments. Or sound-effects (whistling, vocal-chorus, etc.). That's how the film's score comes together. -Written on paper, to be played by Nicolai on the piano. This time containing Morricone's hints about orchestration.
Contracts are signed to reserve an orchestra and recording-studio, to fit the film's post-production schedule.
Nicolai does the 'heavy lifting', or conducting.
As Morricone used to do... until his talents became in huge demand.
And Nicolai eventually composed his own scores and developed conducting-proteges until his death last January.