A 'Phone Companion' app on the PC will help you install the Cortana app from the Google Play or Apple App Store onto your phone so you’ll be able to take the intelligence of Cortana with you, wherever you go.
But Android and iPhone users will not have all the functionality of Cortana. There will be certain things that Cortana does on Windows phones that won’t work on Android devices or iPhones. For example, the ability to invoke Cortana hands-free by saying "Hey Cortana" requires special integration with the device’s microphone, so that feature will be limited to Windows Phones and PCs.
Cortana will be also integrated on Windows 10 PC and Microsoft's new web browser for Windows 10 – Microsoft Edge.
The 'Phone Companion' app we mentioned above will actually work like a hub to direct phone owners to download apps like Skype, OneDrive, OneNote, Outlook, Office, Xbox Music.
The phone companion app will be released as part of Microsoft’s Windows 10 Insider Preview "in a few weeks."