I use Duolingo for learning languages. But for general knowledge I try to read as many books as possible. I found every shortcut I tried so far very unsatisfying. Knowledge from Apps, YouTube or other places on the internet is mostly shallow. I sometimes use blinkist, but if I'm really interested in some topic, I see no other way than to dive into a full book. For general knowledge I would recommend to start with Sapiens by Harari or Behave by Sapolsky. Both are extremely knowledgeable. If you are not into reading, you may try audio books. I used Audible, but it is Amazon and thus kind of evil. 😉
The best way to grow general knowledge is probably too read m I'd recommend making a small goal like a book every two months or something to start with and then work your way up. Goodreads is a pretty good app to keep track of of your progress. If fiction is easier for you, start with that to make reading a habit and then start with nonfiction in whatever topics you find interesting/want to know more about.