HomeKit

HomeKit is a software framework by Apple, made available in iOS/iPadOS that lets users configure, communicate with, and control smart-home appliances using Apple devices. It provides users with a way to automatically discover such devices and configure them. By designing rooms, items, and actions in the HomeKit service, users can enable automatic actions in the house through a simple voice command to Siri or through the Home app. With HomeKit, developers are able to create complex applications in order to manage accessories at a high level. HomeKit is simply a communication protocol, which integrates and operates several types of accessories within the home.
Overview.
HomeKit was created for several reasons. The main reason was to make tasks inside the home easier. It was created to provide people with methods and different tools to change and adapt certain home capabilities to their specific desires. This was done to compensate for the on-going request of user-system co-evaluation. HomeKit manages connected home appliances through the HomeKit Accessory Protocol (HAP). Messages from HomeKit are continuously being sent by powered devices, which are connected to HomeKit. They incorporate fields which recognize the specific accessory and what category it is under. Each category, also, has a code that is used to identify what the device is. It, also, identifies with the Global State Number (GSN). This number is increased each time that the state of the accessory is altered. Like, most apple devices, Apple Continuity Protocols are used. Continuity protocols consist of wireless technologies, such as Bluetooth/BLE and WiFi. They can be conducted through device-to-device connection. HomeKit uses Bluetooth and Wi-Fi protocols. Manufacturers of HomeKit-enabled devices are required to enroll in the MFi Program, and initially all HomeKit-based products were required to include an encryption co-processor. The latter requirement was later changed in iOS 11, which added support for software-based authentication. Equipment manufactured without HomeKit support can be enabled for use through a "gateway" product, a bridge that connects those devices to the HomeKit service. 
HomeKit primarily competes with smart home standards from Amazon and Google. As of October 2019, Apple lists 450 devices compatible with HomeKit, compared to 10,000 for Google and 85,000 for Amazon.
Device categories.
HomeKit currently supports the following device categories (an extended list): 
Garage doors, locks, security systems, and windows are categorized as secure appliances, and requires a device with authentication such as an iPhone or iPad to control.
Home hub.
iPads, HomePods, and fourth-generation and newer Apple TVs can be used as a home hub to control HomeKit appliances remotely, grant guest access, and set up automations. The third-generation Apple TV supports limited features.
Home app.
iOS and watchOS versions.
HomeKit was officially released on September 17, 2014, along with iOS 8. The framework allowed third-party apps to interface with HomeKit devices using Siri and allow remote access through home hubs. 
The Home app introduced on June 13, 2016, unifies all devices into one app and was officially released on September 13, 2016, along with iOS 10 and watchOS 3. It added support for automations using a home hub, and preprogrammed "Scenes", which can set multiple devices using a single command.
macOS version.
The Home app was added to Macs with macOS 10.14 Mojave, released on September 24, 2018.
HomePod and Apple TV.
Fourth-generation and newer Apple TVs can control HomeKit devices using Siri voice commands/ tvOS 14 added direct control of HomeKit devices in Control Center and camera feeds and picture-in-picture monitoring for HomeKit-enabled security cameras.
The HomePod lacks a graphical user interface to control HomeKit devices, and instead use Siri voice commands. Neither the HomePod nor Apple TV can control secure devices.