I kno how she does it. She doesn't have any back up singers but she sings multiple times. She records herself singing the chorus. Then does it again but in another note. Then puts it together and both recordings sing in chords.

Or another alternative if you can't record. Just use the chorus effect. Utada also uses reeverb in all her songs. Most artists do nowadays.

I kno garageband has a simple chorus modifier. You can't do much with it but it's better than nothing. Try FL Studio. Or Reason. I use both.