Writing articles has been one of the best ways I've found to learn. Short posts like this, longer pieces on Medium, internal docs at work, all of it. To explain something, you have to understand it, organize it and say it clearly, and that's where the learning happens.
Every time I write, I learn more than I expected. Explaining a topic, building an example project or preparing a talk for my team makes me dig into the parts I was glossing over. It ends up as documentation for me and for whoever reads it.
You also end up with something useful for other people, practice showing good code, and practice presenting ideas, in writing or in front of your company. But I don't think it's worth writing about just anything. My best posts came from real curiosity, problems I hit at work, or things I enjoy.
For example, I used to struggle with navigation layers in large iOS projects. So I wrote three articles about it, built small example projects and gave a couple of presentations. By the end I actually understood it. The same thing happened with networking: I didn't get how the networking layer worked, so I built a personal project around it, and that's how I ended up learning backend development for the first time, as an iOS dev.
If you're thinking about writing, start with something you want to understand better, or something you wish you'd found when you were learning it. You learn a lot doing it, and you never know who it might help.