Q:

Can I use Siri with these apps?

Could YouTube, Todoist, myEmail (fill in name app your looking for) be controlled/used with Siri?
Could a third party iOS app be controlled by Siri?

A:

Apple is opening up Siri for third party app developers. It's rolling out slowly in which areas Siri could be used. This to ensure the quality. Every iOS major release Apple adds categories where Siri could be used by third party apps.
Siri works in these categories
iOS 11:

To-do lists
Notes
Reminders
Banking for account transfers and balances
Apps that display QR codes.

iOS 10: 

Audio or video calling
Messaging
Payments
Searching photos
Workouts
Ride booking

Recognising
You can recognise which apps support Siri by looking for the Siri icon in the App Store.

