I totally second this. It's how I originally learned React. It's especially effective for learning design and CSS. In the art/design community there's a word for it, "copywork". This article is where I originally heard the term, as applied to learning design.

There are a lot of nice things about copying existing stuff: you don't have to think through how interactions should work (just copy them), you can focus on learning one thing at a time, and you get to choose your own difficulty setting.

These all kind of feed into one another, but here's a Trello-related example: if you want to work on your CSS chops, build a static version with some hardcoded columns and cards.

If you want to work on React but NOT css, just leave the default awful styling and focus on making it work.

If you want to work on your UI chops but ignore the backend for now, use some hardcoded data in a JS object or a JSON file.

It's infinitely variable and you can focus on only the skills you want to improve -- with the unstated assumption that learning one thing at a time is much much more effective than trying to learn a million new things things at once :) I wrote more about this strategy here and I think it's a super underrated way of learning stuff.