Generally if you downloaded it from somewhere/someone you can get a working node from them. Often binaries have a few boostrap hosts hardwired, but then save state once launched. Typically a host tracks several hosts per bucket, and a bucket for each log(n) of the population. So with a million hosts your client will track log(million)*4 or similar. Those are usually written to disk when you close, so you can try those when you launch next.

Clients often broadcast on the local lan to find peers that way.

Also other methods are common, for instance bittorrent clients often use the PEX standard to exchange peers, those peers are often on the same DHT, although I think there might be 2 distinct DHT networks among the popular bittorrent clients.

Worst case you could start randomly walking the IPv4 space, given that there's millions of active clients it wouldn't be long before you found one. Likely only take you a few 1000 random IPs before you found someone on the DHT.

Yes but that doesn't require a central host. If your friend is connected to the network you can join it via them.