Can I create a Siri custom voice command that performs actions in an app? I can't seem to find any docs for this so was wondering if this is possible.  I have a SmartThings app on my phone for controlling my Samsung tv.  I was wondering if I could create a Siri voice command that opens the app, clicks the power button in the app, and then goes to a specific channel.  Not even sure where to begin or if this is even possible.
If you want to open applications that are pre-installed in iPhone then you can add shortcuts for them with Siri. They are provided by apple. If you are using iOS 13 then there is an app called Shortcuts through which you can manage all your shortcuts.

But if you have downloaded any app from App Store then that application must have provided support for shortcuts. If your app is not providing such functionality then you cannot add a shortcut to open that particular app from Siri and perform any action.