To subscribe to a web-based ICS calendar, you can either use your cloud service or 3rd party apps.
Via Cloud Service
For Google Calendar, you can subscribe to calendars by URL here.
For Outlook.com see this help page.
Otherwise, use the cloud service's online help, if you do not find this in the web interface of the cloud service.
Via 3rd Party App
We recommend the apps ICSx⁵ and Ical Import Export. We cannot provide support for 3rd party apps, for questions or problems please contact the respective developer.