How do you build a network? I have a few previous employers, but they don't have any help for me directly, and I haven't had any luck by applying or reaching out to recruiters or agencies.

If you're an individual contributor: keep in touch with colleagues, especially senior level and technical managers, or people going to other companies.

Managers: keep in touch with the best people and just hire them on your next job for more than they were getting paid. Win-win.

It's not easy for everyone, if you don't interact with colleagues outside the job it's probably impossible.