You will continue to use the classic app for pretty much anything that is using custom code. The new app is not supporting those yet. You can use both apps at the same time if you have a reason to need the new one, like a Samsung television or managing the Wi-Fi features of the hub , but otherwise most people are just sticking with the old app for now.
Also, be aware that the “smart home monitor” feature in the new app is completely separate from the one in the old app. Arming one will not arm the other. They are just two different features that happen to have the same name.
