Accessing iCloud
How do I access iCloud on the phone I’m using?
iPhone 5s, iOS 10
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Access iCloud in what way? On an iPhone, if you go to iCloud.com using a web browser you have very limited capabilities. See this for what you can access on the phone.
Which iCloud.com features are available in the browser on your device - Apple Support
However, there really is no need to access iCloud.com on your phone since everything in iCloud is already on your phone. Your files are in iCloud Drive, notes, reminders, calendars and photos are in their respective apps on the phone.