I find this kind of advice underspecified. The people struggling the most to find work are juniors: what projects are big enough that the applicant would a) know and care about them and b) get a benefit out of the network but also c) have fruit low enough for a solo junior to reach?

I tried this way-back-when and ended up submitting fixes to projects that were open source but had no real path to accepting patches from people outside the cathedral.

From the top of my head: Firefox (https://codetribute.mozilla.org), LibreOffice, Gnome, ...