I've seen the DNS seeds in the source code, and I thought "hard-coded seeds" was just another name for these DNS seeds.
So, where do I find these 600 hard-coded seeds?
They only have them for the main network.
https://github.com/bitcoin/bitcoin/blob/master/src/chainparams.cpp#L21Also, regarding these "peers you heard about within the last few days" ... where do they come from?
You can ask your peers to send you addresses. I think nodes broadcast their address every so often.
They use the "addr" message.
That is, how do I hear about them? Is there a list of such nodes on the web or something? Should I be putting these in the bitcoin.conf file with addnode?
No, just being connected to the network is enough. Your peers will send you addr messages every so often.
If you have less than 1000, I think your node will ask for them.
The addr message has the last time the node was seen as part of the message and also if it is a full node or an SPV node.