How do you progress your skills?

I'm not new to programming, but I'm not an expert in a specific language either. At the company I work for, they do something they like to call "rotation." Rotation refers to being placed on different projects and never exactly staying within one area of expertise. In other words "rotating" between different groups/teams that focus on different things.

One project I may be using Java, and in the next project it might be something with C#, C, Excel VBA, etc. These projects usually last a few months. As someone who likes to learn new things, the upside is that I can gain some background on various languages, environments, etc, but the downside is I don't get to completely focus on one language, thus making it difficult for me to become an "expert" in anything.

I've started studying Kotlin (really want to learn it) on my own with udemy.com courses and also building my own Android app. On top of that I started a MERN stack project with my friends so that we could learn it together (With the aim of putting the code on GitHub).

Besides these two examples, do you guys have any advice on how to improve my skills? I would love to continue working as a developer in maybe a different company, but my current situation doesn't exactly provide "proof" I'd be able to keep up in a company that might specialize in (a) specific language(s).

Thanks for reading.