What to do to learn + stand out

I have some downtime coming up, what can I build that will work to impress? Should I just do open source?

I don't think there is a cheap way to impress the interviewers. Any fake project that is just written after watching YouTube video is obviously fake. Yes, you can change titles, you can add more functionally, but it is not a real project, nobody uses it. Interviewer will see it. It's good that you take some courses to learn the subject, but do not think too much of those. Yes, open source is a way to get valuable experience and recognition, but it will take time. Do not expect that you fix a couple minor issues in some popular software and they put you on a list of top contributors. At the very least it will take many months before your open source contributions might be called as something important. If I were you, I would just learn the language, practice leetcode and go for interviews. Do not look for cheap ways to impress people. Good understanding of main algorithms and data structures will give you much more points on the interview. That's it, it is that simple. I did many interviews in my life. Both as an interviewer and the candidate. I never paid much attention to any non paid hobby projects. And me myself was only asked about details and experience of the most significant projects in my career. The rest of my resume was normally just quickly looked through.

I was wanting to start working with Swift soon, what made you pick flutter?

I want to eventually make the same app in Android and didn’t want to use React Native for whatever reason.