In my first software job I didn't have enough to do all day, my defined responsibilities were less than 15 hours a week. I found other useful stuff to do, but there was still a ton of downtime. This was in 1999, I had no phone to scroll. I read "Inside SQL Server" cover to cover (400 pages). This was a huge lift to me, I understood so much more about writing efficient queries and tuning - I was nearly fresh out of college and better at SQL than anyone else in my department. I'm
still benefiting from this today, even though I haven't used SQL Server itself since 2008. New knowledge latches onto existing knowledge, and the gains accumulate your whole career.
I'm still in the habit of reading books when I pick up a new technology. It isn't necessary to get things done - I've never read a book on python but can still get things done in it - but making the book bedtime reading has been really effective. I don't try to memorize or apply everything, or even do the exercises. I just read it, and even if I don't remember the details, I'll know the features are there. How else will you learn what you don't know that you don't know?