Unable to access iTunes library in slideshow using iPhone 15 Plus

I've been trying to add purchased and downloaded songs from ITunes (which I literally have thousands of), not from the music service. When I click on the little music icon at the bottom of an existing slide show all that is available for me to choose from is the canned songs in the picture app (which are crap btw). I have no option to select my library so I can access my own music. How do I make my library visible? Any help would be appreciated!