After the Basic Tracks have been recorded, the next step in the process is usually the overdubbing of various other instruments.
The overdubbing process has expanded quite a bit with the advent of endless digital tracks. Typically rhythm enhancements are added, with various percussion instruments and additional guitars. Sometimes horns and strings are added to the recording.
The vocals are usually a main part of this process. Selecting the very best mic for the lead vocals takes some time. Then working out the background parts can take a long time. Harmonies, doubling of parts, adding shouts and other emotional components to the song has taken on a new meaning enabled quite a bit by the new technology.
Getting a good headphone mix is an important part of inspiring the performances. Adding solos and other rhythm instruments is something that a producer usually decides. The overdubbing process can take some time as different artists are available at different times during the process. It is often the case that the primary artist was never in the room with any of the musicians.
Sometimes parts are added that are not used, and often musicians are playing duets with other musicians they have never met that have recorded tracks on the other side of the world. Such is the state of the technology. The job of the producer through all of this is to keep the vision of the song alive through the disjointed overdubbing process.
There is a lot of creativity involved during this step as the energy and unique sound of the recording can be greatly influenced during this process. One of the main tools afforded by new technology is the ability to put off the decision-making process until later. It’s not like George Martin and the Beatles having to make final decisions at every step since nothing could be undone once it was committed to tape. Now there are unlimited tracks and nothing has to be decided until the final mixdown.