Bitcoin Core is the software. Core can work as a Wallet and/or a Node. Same software, two different functionality that can work at the same time, or not. Does this answer your question?
I don't think there's a way to limit bandwidth usage in Core, you'd have to do that at the system level I think.
What settings do you have to alter to make Core work as a Node? I know wallets and nodes operate differently, and that you need to open port 8333 to let your node get more than 8 connections to other nodes, but there must be more settings that need changing. Otherwise everyone' wallet would operate as a node.
None, Core has the ability to work as a node straight out of the box. Everyone's wallet is a node, you just have to forward the right port and you're contributing.