Depends. First one was a copy of something I had seen work, but integrated with a different service. Second (higher grossing one) was just something I noticed in the marketplace that was getting bought but didn't work very well. A third app was a copy of the second, but a bit different, which also did well.

You'd think all low hanging fruit is gone, but I don't think this is the case. It doesn't need to be an original idea if the alternative isn't working very well, for example.

One thing I can tell you: there are a lot of opportunities you miss because you're not paying attention and busy with work or other random things.

I personally wish I had more industry specific knowledge so I could go disrupt things, but getting up to speed in an industry is a lot of work if it's not something you're at home in.

I also read a lot (and probably too much)

Stay curious and well read and ideas will come to you. Well read, to me, implies reading about advances in tech, in culture, and in environment. Reading a lot of books is great for synthesizing crossovers.