How does song notation work? What is a song? What do you need to write down in oder to notate a song?
As is the case in many creative endeavors, the answer is relative.
If you want to create a lead sheet, you notate the melody, lyrics and chords. You add some information about the tempo and mood, and that's about it.
But, what happens when you have a budget and players are expensive and studio time is expensive, and you need to communicate with musicians quickly and effectively?
Then you need detailed scores.
What if your song is for piano and voice? Then you have a whole history of song notation to draw on. You can copy scores from great composers of the past, and you can learn by following in their footsteps.
If you are interested in what I've devised as an efficient way to learn song notation, I recommend my eBook "How to Write Your Songs Down!"