The lie of the one thing
Focus on just "one thing." Isn't that the popular advice? Yet, we conveniently leave out what we mean by "one."
Ages ago, being an engineer used to be "one thing." Then, being a software engineer became the next "one thing." Then, a backend engineer. Then, a backend Java engineer specializing in relational databases. Ad infinitum.
Here's another option: instead of debating what's really "one thing," just learn whatever interests you. In the end, the only "one thing" that matters is you.