My point is: if you redesign stuff, you need to find out what the problem is they've solved. E.g. search the whole music catalog with a touch device. You also have a solution with wich you can compare your work. If you work for something, you do not have a solution to compare yourself with, it's more difficult to learn.

I'm not talking "rebuilding or copying" without questioning - that has nothing to do with design - that's indeed just learning the typewriter by typing a books pages. I'm talking redesign. Which means imo: solving the same problem in another way. That's how I understood your reference on rebuilding Apples App Store, or Spotify, Uber, Whatever. There are constraints why big corporations build their solutions the way they do. Even if we as designers think, that could've been solved better.

If you work on a new problem, you might miss a lot of points, which you might learn from redesigning for example Spotify. But I say "you might". For sure you can learn a lot of stuff while working - also for non-profits. I found it for my students more helpful to go the "Jared Spool" kind of way. Loved his Boarding Pass example. IMO one of the best possible way to show design-constraints that are not obvious.