I had high hopes when I tried it out but frankly it's been almost unusable for me. Terrible performance, laggy UI, plenty of bugs, long loading times for songs...
I don't know if something in my mobile environment was messing with it but I use quite a few indie FOSS apps still in beta and none of them worked as badly as Spotube did. I'd love to go back to it if it improves, but for now it's just not worth the UX pain.
Edit: forgot to mention. The idea of sourcing tracks from YouTube is cool but causes loads od trouble in practice. I've found remixed versions streamed as the original, tracks with the intro from the music video, tracks with sound effects from the music video, and tracks that just cannot be streamed cause they aren't on YouTube. I know there's a feature to pick which version to stream, but it's quite a bit of UX friction and it didn't work often enough to be a showstopper.
I believe you need root to access those, plus a file manager that supports it (I use Mixplorer which does). Otherwise, as someone else suggested, you can access them from a computer over ADB or MTP.