Solfege to remake songs?

Hey guys I've been told solfege will help me play a melody by ear and have been learning about it. One thing that really confuses me is do I approach it by using intervals to remake the song for e.g hearing a melody n hear it go to a minor 3rd up then a major 3rd up and so on n so on or do I internalize scale degree and what each one sounds like and for e.g when I hear notes i know that's the 4th scale degree to 5th scale degree etc?