Is there an app that could identify the Bass notes in any given song?
I will earnestly suggest that you get your ear trained up to hear it on your own, as there is infinite power and knowledge and skill and musical prowess gained from personal transcription!
And I know @JoshFossgreen has a line on some killer transcription tools…
AFAIK, no. The process of transcribing bass lines is so complex, requires attuning to so many variables, with a huge breadth/depth of the instrument, the instrument’s history, and tones within a certain genre… I don’t think a piece of software could do it. Maybe it could get you 70-80% of the way. But unless you have good ears, you wouldn’t be able to check its work for errors, omissions, or stuff from other instruments that got artifacted in.
Sooooooo there’s still no replacement for ear training!
It won’t do it for you, but there’s a program I like called Transcribe! that has a bunch of functions that can help you with your transcribing - speed/pitch controls, looping functions, making section markers, etc.
And musictheory.net has a bunch of great ear training exercises you can do for free: