The word "home" appears in any discussion too many times with too many meanings!!
Starting with your iPhone or iPad, there's an iOS App called Home. It's usually preloaded, but if you're not running a recent version, you'd have to get it from the App Store. It's Free. The iOS app Home, works with the iOS framework and database called HomeKit. Within the iOS app Home, you will find tiles for your HomeKit devices. You can organize them into Rooms and create Automations, including presence.
iOS Home app Automation however, requires a HomeKit Hub. You can use an Apple TV, an always powered on iPad or a HomePod speaker. All of them simply need you to login using your Apple ID and tell it you want it to be a hub. For Presence, you need a the HomeKit hub.
The next component is Homebridge which runs on a computer that is on your LAN. It can be a Raspberry Pi, or Linux (like MacOS) and there is a Windows version.
The last component is the Homebridge App for Hubitat. Like a lot of apps, it is where you tell it which of your hundreds of Hubitat devices you want to see on your iOS Home app. This is all very similar in data flow to Google Home or Amazon Echo... but Apple.
Homebridge is a bridge. It gets a message on 'one side,' transcodes it, and sends it out the 'other side.'
The Homebridge App for Hubitat will see that a light got turned on.. and sends a message to Homebridge which converts it to a HomeKit message and sends it to the HomeKit Hub. Your iOS device will see that the Light is on, and will change it's tile to match.
The reverse is true too. By creating an Automation on your iPhone or iPad, your iPhone becomes a presence sensor and when it comes within Bluetooth + LAN range, the HomeKit Hub will know and send a message to Homebridge. It will convert the HomeKit message to a Hubitat message and tell the Hubitat hub,
Once working, it all goes 'invisible' just like Amazon Echo and Google Home, but with a presence sensor none of those have.
You can ask Siri to turn on lights, open shades, shut the Garage door, etc. Presence is in the background and all it does is turn on/off a Hubitat Virtual switch.