What do I need to buy to have HomeKit

You already have HomeKit on your HomePod. You just need to purchase smart devices that are HomeKit compatible. Apple also now supports the new open standard for smart device connectivity called Matter. Any device that is Matter compatible should work with HomeKit now. For devices that don't support either you will need some kind of bridge that converts that device to HomeKit. Phillips Hue products were notorious for requiring their proprietary bridge to work.

According to Logitech’s support document for your door camera it is HomeKit compatible so it you just need to follow the setup steps in the camera’s manual.

Homekit is a process/situation/framework/whatever. You use the (already installed) 'Home' app to interact with Homekit devices.

No clue Siri tell me I need HomeKit for whatever question. I asked her Google voice 10 times better. This is the worst iPhone in history. My keypad started talking this morning. I go through all the directions are supposed to everything is turned on or off in the settings, it still does it, I can’t find HomeKit is it