For iPhone: See: Use the built-in privacy and security protections of iPhone - Apple Support
Be sure to use Two Factor Authentication for extra protection and you need this to set up a HomePod hub
I think you meant “layman’s terms” but there is so much to cover it would help if you specified what you don’t get. I suggest you start with the below and after that it is more or less learn by yourself until you get the hang of it. Then you can come back and ask for help with specific areas.
As to HomeKit you will need an Apple tv box or HomePod to start. This is so you have a Hub. Then you will need to use smart accessories that say works with Apple HomeKit. For the most part they run best on a home wifi network and all should be signed into the same network and same Apple ID
Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support
Disregard it saying an iPad can be a Hub, it no longer can. A HomePod mini is the cheapest option to start with and we can help you set it up if you can’t do that.
