It all comes down to NAT and dynamic IPs. Let's say Anu and Bob and Chen are trying to play a game. Anu runs the server and wants Bob and Chen to connect to her

It all comes down to NAT and dynamic IPs. Let's say Anu and Bob and Chen are trying to play a game. Anu runs the server and wants Bob and Chen to connect to her server. Anu follows some guide to get her IP and gives it to the others, but it's not routable publicly because of some form of NAT, or her IP is dynamic and changes and so packets become unroutable to Anu. Now all Anu needs to do is press "Start Game", where a remote server either hosts the game metadata or just acts as a glorified STUN/TURN server and the remote server gets a friendly name that Bob and Chen can use to connect to the game. If IPv6 and NAT was not a thing then IP LAN games would have probably had no trouble. Factorio still does multiplayer through direct UDP/IP, so you can definitely play it online purely IP P2P. I've played Factorio games served using ZeroTier before with no issue and it's allowed me to host Factorio on my laptop from both home and a hotel.