Also: What is Security Keys for Apple ID and why does it matter? With that information, you can then create automations in the Home app to do things like control your HomeKit-connected thermostat or turn a fan that’s connected to a smart plug on or off. 
In order to activate the sensor, you’ll need to do two things: First, install HomePodOS 16.3 on your HomePod Mini. Second, you’ll need to install iOS and iPadOS 16.3 on any devices you want to see and use the readings on.  Both processes are simple, with the HomePod update taking the longest. Below I’ll walk you through the steps. 
Once your iPhone or iPad is updated, you’ll be able to see the corresponding readings in the Home app. Below are a few screenshots of what the readings look like for the HomePod Mini I have in my office.  Your HomePod will stop playing any music and become unresponsive during the installation process. Also: How to add your HomePod to the Home app and Wi-Fi Install the update by following the standard process of opening the Settings app and then going to General > Software Update.

Can I turn off the temperature and humidity readings? 

As far as I can tell, right now it’s not currently possible to remove the temperature or humidity reading from the Home app without also removing the HomePod Mini from your home.  Review: The HomePod Mini is for Sirious Apple users only Thankfully, they aren’t very prominent in the Home app and you can just ignore them. 

Does the original HomePod have the same feature? 

It does not. Only the HomePod Mini and the second-generation HomePod that Apple just announced has the sensor.