Dedicated Server is just a renderless client / target.
IOCP is an API.
Gamelift is a Hosting service using AWS and has some scaling features.
MMOs are all dedicated servers (no one is running a listen server as an MMO). You likely aren’t going too find any server running 10K connections, instead they have a farm of instances running across multiple PCs and use load balancing to distribute players across the nodes (pretty much what Gamelift describes).
I’ve worked on three shipped AAA MMOs - they are behemoths of tech, hardware, and content. I’d highly suggest you aim at just making a fun multiplayer game and THEN you can look at scaling it up for more players as you get more comfortable with networking.