10 ways to expand your professional network

Go to LinkedIn, find people that are doing cool stuff and see if they are available for a virtual coffee

Ask your warm contacts whether they can introduce you to others. Offer to introduce them to people they are interested in.

A lot of them may be remote so you don't even need to be in the same city / place

I'm sure you have a stack somewhere you've saved over the years. Go through them and connect with them on LinkedIn.

If you're on LinkedIn, it'll have to be work-related but write articles sharing what you know / learned / mistakes / opinions.

Although I wouldn't say add everyone you have interacted with, I would say to add those people you have had a good interaction with - maybe you've talked about personal things, maybe you worked on a project together, etc.

I like to kick things off by writing up recommendations for former coworkers. Sometimes they'll return the favour. Sometimes they won't (but that's okay).

Again, the idea is to give without expecting anything in return.

See if you can find in-roads into those companies through different contacts. Maybe it's a friend of a friend. Maybe there are info sessions where the recruiter can put you in touch with the right people.

If you meet someone and you don't know if you'll interact with them again / have a chance to build up a relationship, ask them if you can add them on LinkedIn. Build it up from there.