There are many good things mentioned here already, and I would like to add one that might not seem that obvious at first. Teaching. I once read a book (about C#) that had the following line in it: "Qui docet, discit - He who teaches, learns". This line stuck with me since then.

Now, many years later, I can reaffirm this. Teaching others something requires you to understand whatever you are teaching them, and even though you think you already understand something, when you teach it you force yourself to communicate it in an understandable way to others. In addition, you sometimes will have to do some more research to learn more about what it is that you will teach.

I started blogging about a decade ago, mostly about things that I found were interesting and then decided to actually dive deeper into the subject, and then write up a blogpost to explain the subject in understandable pieces.

Later in life, at university, I did some teaching of students whom struggled with several programming courses. Once again, I learned a lot from this. Sometimes they had questions that I never would have considered and then had to look up, to explain it to them in the next lesson.

And now in the industry, where I work, we give "devtalks", we can talk about anything technical we want and teach it to the others (whomever attends, but usually attendence is about 50-60 people). I like "going deep" in technical things, so I force myself to "go deep" and learn more about what I will talk about and then once again, present it in an understandable way.

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This is maybe not the thing that let me grow _most_, but I would certainly say it is a valuable addition on the road to become a better developer. And besides, the other commenters already hinted that there is no "single thing" that let's you grow most ;-)