I've honestly never found myself in a position where I didn't had some spare time whilst at work. Even at my current job, I've transitioned from frontend work to backend work (I had backend experience already in Node), and I've always been able to keep up with our sprint. Other engineers complain they don't have time to improve our code base, or they can't learn anything new because they're always busy, but I think that's just an excuse. It's just a matter of organising yourself and looking for opportunities to learn.

I actively picked up stories I wasn't familiar with, just to learn it. Other engineers pick up stories because they're already familiar with the subject, that's not how you learn new things.