If you're at home, controlling your HomeKit enabled smart tech, then you'll have no problems at all doing it from within your Home app on your iPhone or your iPad.
However, if you want to turn your lights on and off, check your air quality, activate your security alarm or have remote access to HomeKit enabled cameras, when you're not at home then you'll need a HomeKit hub to run the show. But the good news is you don't need to buy a new device. Your HomePod, Apple TV 4K, Apple TV (4th generation), or iPad (with iOS 11 or later) will do the job.
Why you should use Apple TV for HomeKit
Before the HomePod came along, HomeKit hub dibs was a straight shoot-out between an Apple TV and an iPad; and the former obviously held the trump cards as it won't run out of battery. The HomePod also has that advantage over its iOS brethren too.
And that's a biggie because, the whole reason for having an Apple Home hub is you have a device that's constantly on and always able to send signals around your house - whether those signals be Wi-Fi (Lifx bulbs, Philips Hue Bridge) or Bluetooth (Elgato Eve, Fibaro Flood Sensor). As long as your iPhone has a data connection, when you send an action through the Home app, your hub will be able to pick it up over the internet and carry out your instructions in your house.
When using Bluetooth, be aware that your devices will need to be close to your Hub. However, multiple devices can be used as hubs so, if you've got an Apple TV in the bedroom and a HomePod in the living room - that's a good thing.
Having a HomeKit hub set up also lets you automate actions based on your locations. As long as your iPhone is running iOS 11 or later, your HomeKit hub will carry out automations you've drawn up that are dependent on you being home or away. Just make sure that you (and other users in your Home group) have location services switched on.
Setting up your HomeKit Hub
Using an Apple TV (4th-gen or 4K)
1. On your iOS device enable two-factor authentication for your Apple ID. Also, go to the iCloud settings menu and check keychain access is enabled.
2. Sign into the same iCloud account on the Apple TV ('Settings' > 'Accounts' to check).
3. iCloud should do the rest but you can go to 'Settings' > 'Accounts' > i'Cloud' to check if HomeKit is connected. You can also check the status of your Home hubs by tapping in the right-hand corner of the Apple Home app screen.
Using a HomePod
Like the Apple TV option, a HomePod makes a lot of sense to use as it's likely to be static in your home, and powered up all of the time.
Like Apple TV, HomeKit hub setup on the HomePod is an automatic affair - just makes sure you've enabled two-factor authentication for your Apple ID, approved keychain access and signed into the same iCloud account.
Using an iPad
Ensure you're signed into your iPad using the same iCloud account as the rest of your iOS devices.
Go to 'Settings', tap on 'Home' and select 'Use this iPad as a Home Hub'.
Open the Home app and tap in the upper-left corner. Then look under Home Hubs to see if your home hub is connected. If you have multiple home hubs set up, the primary home hub will show as connected. The other home hubs will show Standby as their status and will take over remote access if the main home hub is disconnected.