I’m looking for software (including a music library) that makes it possible to play original music tracks (rock and blues from 60’s to now) split to single instruments (one instrument - one track).
Would Steinberg Cubase help ?
This is not possible with any software. You would need to have the multitracks or stems to do this.
Have not tried it myself, but check out Spectralayers Pro
Guitar Pro 7.5 by Arobas is a great practice tool for guitarists. It’s a notation software but there are loads of scores you can download either by purchasing them individually or with a subscription. You can get free scores, riffs and licks on the website. Plus you can learn notation and write your own music. There’s a free trial download.
You can get pre-recorded tracks split up into individual instruments - check Jam Kazaam anbd similar
How does JamKazaam do that? I thought it was a live online jamming thing
It is the tracks are pre recorded
The OP is asking about taking existing recordings and “un-mixing” them, not about where they can find generic backing tracks.
Well I( know that, but if he is a newbie practsing a guitar he might not know teh right questions to ask. He can ignore my posts and so can you. I also suggest he takes a look at Band in a box, that will help him
Yes, I’ll give Spectralayers Pro a try 
thanks