You can buy a course for all platforms (Web, Android, iOS), or upgrade to all platforms a purchase done on Google Play or App Store. The purchase of a course give you access to that course forever. Subscription to a course of group of courses gives you access to those course(s) on all platforms (Web, Android, iOS) for the duration of the subscription chosen.
Subscribe and get access to all courses in the package on all devices.
You can study the courses on mobile devices - just install the Chess King app and log in using your account details.