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