Can I create a Siri custom voice command that performs actions in an app?
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.