If I want to learn a language, I'll find a basic book on it and then grab as much native audio as I possibly can.
If I want to learn about something in math, I'll go to Khan Academy.
If I want to learn about irrational decision-making in everyday life, I'll read a book.
And that all starts with the assumption that learning follows the waterfall method: pick what you want to learn, learn it, move on to something else. I find learning much more iterative: be intrigued by something, follow it, see what it leads. I never really decided to learn about systems thinking, but I came across the personal MBA reading list one day, thought it was fascinating, came back, picked a book that looked interesting (Thinking in Systems), and read it.
A couple of weeks ago, I learned a great deal about writing fiction with characters of a different gender or race than you. I never set out to, but I do NaNoWriMo each year, and one of the most active posts on their forum was about "writing the other." It fascinated me, so I followed it, and learned something for it. This, to me, is what lifelong learning looks like.
That said, I didn't want to limit it to a specific topic (e.g. just learning a new language) and hoped that people would enumerate some of their preferred topics and corresponding methods, just as you did :)
I was also curious about how people go about choosing what topic to dive into next, if it's anything beyond the normal ongoing mental list of interesting potential topics.
As for how I use the software, as I'm reading something I want to retain, I try to extract essential concepts as question/answers pairs and add them into a deck in the software. Then review the deck with the software on a regular basis (usually daily). The spacing effect provides maximum learning with a minimum time investment.
As for choosing a topic, that's entirely up to your personal interests. There are no shortages of posts on Hacker News listing books the readers are interested in. Search Hacker News for "good books", go through one or more of the lists, and choose one (or more) of the books that sound interesting to you, and get started.