This video is by Don Chandler (Donstrumental), an excellent touring and teaching reggae bassist. His approach can be applied to any style of music. IMO, what is most useful is to pay attention to what info he derives by ear from the demo track: song key, chord number and progression, song structure, etc. These provide answers to the common questions of what ear, playing and fretboard knowledge skills are important for creating our own basslines or coming up with a bassline for a song we are asked to play, but don’t know. But in the end, it’s just relying on your training, and just reacting without overthinking it.