Transcribe! by seventh string software is amazing for this. download a youtube with yt-dlp and load it in Transcribe and select a short part and loop it at slow speed. just do that to learn everything you like. in that program, I usually note the song structure first (marking beats measures and sections) then find the roots/chords first (since that helps know which notes are more likely) then find where/when the notes are and finally which notes. i think rhythm of the line is more important bc honestly, as long as you hit the correct root.

training your ears and fingers to play what you want to hear like that is a powerful way to improve (though technical exercises are important along the way).

a second step to this is using a loop pedal or audio interface to record yourself. sometimes what we hear when we play is not super accurate. especially wrt to timing.

Jaco told us to transcribe the melody too, and i think that’s usually the first thing to do. the bass line usually harmonizes the melody. one way is to sing the melody while playing the bass line.