How does Arrow or Notes System works? (Give technical explanation)

[Poinet]

I am a programmar, I was very interested in how the arrow or notes system works in Friday Night Funkin. I would like a technical explanation (i.e how arrow or note system was programmatically made. What they use to achieve this, like arrays, etc). As well as holding notes, how was a holding note programmatically made?

A technical explanation is appreciated, always help out other programmars, you know haha.

[Snirozu]

when a player enters a song, the game generates notes and adds them to the unspawnNotes array that is sorted from lowest to highest note song position
after it's generated a update() function is called every frame that:

1. checks if first note in unspawnNotes is 1500ms ahead of current song position and if it is this note is moved to the notes array (the notes array renders notes)
2. calls keyShit() function

the keyShit() function (basically input system):

1. sorts notes array from the lowest to highest note song position
2. gets the first note in notes and checks if a key was just pressed for it and if it is this note gets judged
3. iterates through sustain notes in notes and checks if a note can be hit (which should be equal or lower to current song position) and if a key for this note is pressed, if it is this note gets judged

i skipped some less important shit but basically this is how input system works