Where can I open my iCloud?

On an iPhone, iCloud is designed to be viewed by using the apps on the device (e.g., the Files app for iCloud Drive, the Photos app for iCloud Photos, the Notes app for notes, etc.). A Mac is similar except that iCloud Drive is accessed via the iCloud Drive folder in Finder.

You can also use an internet browser and log into icloud.com. What you can access via a browser varies by device. See the information in the following Apple article: Which iCloud.com features are available in the browser on your device - Apple Support

Also note that while you can view synced data as described above, it is not possible to view the contents of an iCloud Backup of an iOS device. You can only use the backup to do a full restore of a device or to setup a new device.

Apple also has an article about managing iCloud storage. See: Manage your iCloud storage - Apple Support