A song has its beginning. When a musician or a songwriter will think of something to be the main topic of the song, it would soon form to have lyrics. As the song can have a chorus and stanzas so they were written down and they could be rearranged or edited. It might not be once but editing could take a few times before the song is finalized. The songwriter is the ones who write songs and they could sell them to singers for performance.
Songwriters could be an individual who sells her own works or they could be one that is employed. A musician or a company can employ songwriters that would write songs for singers whether they are a group or for one person. Not all singers can write their own songs and so they could buy a song from songwriters. They then acquire the rights and so they can record them and perform them. They will then have the right over the song. Their name would be tied with the song.
The song could become very popular especially when they are sung by famous singers. Singers could just receive songs for them to sing as was instructed by their agents. That is why others can sing a foreign song even if they do not understand it. Singers who are also songwriters mostly write their own songs. There are also those who can play a musical instrument and so they can have their concert with them playing a musical instrument.