To get music that plays constantly it's pretty simple. You have to have an AudioStreamPlayer2D or 3D and set it to what music you want. Then you have to make a variable which you can either call from the node path, or use export to make it a public variable so you can drag and drop your music to it. I have a GameManager script where I keep all stuff like this but I *think* you can put it in any script that runs on game launch.

This is how I made mine
@onready var background_music: AudioStreamPlayer2D = $"../Audio/AudioStreamPlayer2D"

Then in your _ready function, call the music
background_music.play()

If you want it to play or stop at different times you can do that with if statements.

To make it loop, select your AudioStreamPlayer and in the inspector check "Autoplay". Then on the left switch from node to import. There is an option called loop which you want to change from default to forward (you can do others but forward is the most common).

Hope this helps, I had to figure it out too as I am also new to Godot.