Wouldn’t that largely depend on what the topic is?
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.