You can start building a network by reaching out to alumni, former colleagues, open-source contributors for projects you're contributing to [1], etc.

Hardly ideal, but it's a start.

[1] And if you're not contributing to an open-source project, please do it, it's a great way to learn stuff, improve your CV, network and of course give back.

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.