How to add seed nodes in altcoin?
The domains listed in chainparams.cpp are for DNS seeders. DNS seeders are not nodes themselves, but rather are DNS servers that serve the IP addresses of nodes that are available to be connected to. These can be connected to for both a normal connection and just one to retrieve more IP addresses of nodes.

The IP addresses listed in chainparamseeds.h are the IP addresses of nodes that can be connected to. This connection can be an actual connection or just one which retrieves more IP addresses of nodes that can be connected to.

To setup a seed node, you just need to run a normal node for your altcoin and add its IP address to chainparamseeds.h. There should be a script that actually constructs this for you in contrib/seeds.

To setup a DNS seeder, you will need special software to do that for you. You can start with this software and modify it as necessary to work with your altcoin.