Do artist eg, Michale jackson put the music, the lyrics etc, all together on their own or do they get help?

A little vague but Im trying to understand how this works.You have a
singer, they write a musical piece, they perform it and in some Cases
also create write the music to accompany this too...
What about in full length various songs like micheal jackson. How is
this done?
Does he hire a band tell them how to do it, or do they just do it how
they think....does he write this, or do they, if it is them, do they
get credit for this....how does all this stuff in making a song
complete work? Who does what, and how does this get sorted?