I had very few job offers originate from FOSS (maybe 2 in 4 years?). I suspect there's a narrow field where people recruit using FOSS/GitHub and I'm not involved in it. JavaScript based projects seem to be doing much better from an outsider perspective.

I got more out of having a decent looking LinkedIn account.

In my opinion... LinkedIn gets you in the door, your Github profile can get you hired. I'm not sure how many job intros happen via Github to be honest... just from my own experience, I get a ton of calls from LinkedIn... and on the interview side, some higher profile contributions to open-source can help a lot.

For where I work now, our code challenge includes putting the solution on github/gitlab mostly to ensure they at least know git, but as a secondary to their gh profile, which leads to their dev and contributions.

It's not a hard requirement to me. I've known a lot of people in banking and secdef development that cannot participate in open-source, so there are definitely all types.

I read it more like "get involved in FOSS projects to gain experience and trust, and base your CV (+ references) on that when applying for the jobs you want".

I've had quite a few job offers from my weblog over the years, most of them actually pretty decent from HN readers and such (rather different from LinkedIn and recruiter spam). Never from open source thus far.

What kind of open source projects are useful to go for? This seems like by far the best way to spend a few months. I know Python best but I don't mind learning a new language.

Projects that businesses use to do something, or aid in doing something, that makes them money.

Okay. How do I go about finding such projects?