Using an iPad as a HomeKit screen
How do I use an old iPad as the screen for HomeKit devices, such as a camera?
HomePod mini
HomeKit isn’t supporting iPad as a hub with the current Home architecture, as that requires an Apple TV or a HomePod or HomePod mini.
Your iPad can be used with the older Home architecture, and can be used with the Home app (though not as a hub) on newer versions.
Here is how to: Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support
You won’t be able to use the iPad with the newer architecture, or use it with Matter support, AFAIK.